In the
 United States Court of Appeals
              For the Seventh Circuit
                        ____________

No. 08-1019
THOMAS SPRINGMAN, individually and on behalf
   of all others similarly situated,
                                     Plaintiff-Appellant,
                              v.

AIG MARKETING, INC. and ILLINOIS NATIONAL
   INSURANCE CO.,
                                  Defendants-Appellees.
                    ____________
           Appeal from the United States District Court
                for the Southern District of Illinois.
           No. 07-737-GPM—G. Patrick Murphy, Judge.
                        ____________
       ARGUED APRIL 4, 2008—DECIDED APRIL 15, 2008
                        ____________


  Before POSNER, ROVNER, and EVANS, Circuit Judges.
  POSNER, Circuit Judge. The plaintiff filed this class action
suit in an Illinois state court in July 2003 against AIG
Claim Services (AIGC), a company that processes insurance
claims, as well as against Illinois National Insurance
Company, which issues insurance policies; the policies
give rise to claims for payment. The complaint charged
the defendants with having violated Illinois fraud and
consumer protection law by underpaying accident-insur-
2                                                No. 08-1019

ance claims submitted by the plaintiff and the members
of his class.
   In December 2003, in response to an interrogatory
served by the plaintiff, AIGC stated that it had not handled
his insurance claim. But not until October of the next
year did he serve an interrogatory asking who had handled
it. AIGC and Illinois National responded the following
month that the claim had been handled by an affiliate of
AIGC called AIG Marketing (AIGM). Three years (less two
months) later, and thus nearly four years after first being
told that he had sued the wrong party, the plaintiff moved
the court for leave to amend his complaint to add AIGM as
a defendant and drop AIGC. The court granted the motion.
  In the meantime, however, Congress had passed the
Class Action Fairness Act, which allows certain types of
class action suit filed in state court and governed by
state law to be removed to federal district court despite
the absence of complete diversity, 28 U.S.C. §§ 1331(d),
1453, provided the suit was “commenced” on or after
the Act’s effective date, which was February 18, 2005. Both
the plaintiff and Illinois National are citizens of Illinois,
though neither of the AIG companies is. AIGC therefore
could not have removed the case to federal court because
the suit against it was filed before the Act’s effective date
and thus at a time when complete diversity was re-
quired for removal of a diversity case. But AIGM was
substituted for AIGC after that date. So, promptly after
the amendment of the complaint (and incidentally before
a class had been certified), AIGM removed the case to
federal district court, which denied the plaintiff’s motion to
remand to state court, precipitating this appeal under
28 U.S.C. § 1453(c)(1). The question we have to answer
is whether the substitution of AIGM for AIGC was the
No. 08-1019                                                  3

commencement of a suit against AIGM within the mean-
ing of the Class Action Fairness Act, thus enabling re-
moval of the entire suit. 28 U.S.C. § 1453(b).
   That the suit had been filed before the Act’s effective date
might seem to doom removal. But the plaintiff does not
argue that; he bows to our case law, which rejects the
position that nothing that happens after suit is filed can
affect removal. E.g., Phillips v. Ford Motor Co., 

, 786-88 (7th Cir. 2006); Knudsen v. Liberty Mutual Ins.
Co., 

, 806-07 (7th Cir. 2005). All but one of the
other circuits to have addressed the question agree with us.
Smith v. Nationwide Property & Casualty Ins. Co., 

, 405-06 (6th Cir. 2007); Prime Care of Northeast Kansas,
LLC v. Humana Ins. Co., 

, 1285-86 (10th Cir.
2006); Braud v. Transport Service Co., 

, 803-04
(5th Cir. 2006); Plubell v. Merck & Co., 

, 1071-72
(8th Cir. 2006). The outlier is the Ninth Circuit. E.g., McAtee
v. Capital One, F.S.B., 

, 1145-48 (9th Cir. 2007).
  The cases that allow removal under the Class Action
Fairness Act on the basis of certain events that take place
after the case was filed are consistent with federal removal
doctrine; “an amendment to the pleadings that adds a
claim under federal law (where only state claims had been
framed before), or adds a new defendant, opens a new
window of removal.” Knudsen v. Liberty Mutual Ins. 

, 411 F.3d at 807; see 28 U.S.C. § 1446(b). The Ninth
Circuit, however, believes that a simpler approach, one
that disregards post-filing developments, is warranted
because all that is at stake in removal under the Class
Action Fairness Act is whether the same case, governed
by state law, will be litigated in a state court or in a fed-
eral court. But that is all that’s at stake whenever a suit
is removed to federal court on the basis of diversity.
4                                                 No. 08-1019

Nowhere does the Act state or suggest that removal
pursuant to it is more limited than in the usual diversity
case. On the contrary, the Act lengthened the time
within which a normal diversity suit can be removed, 28
U.S.C. § 1453(b), as well as making a particular class of
diversity cases, namely class actions in which diversity
is only partial, removable to federal court for the first time.
  On the Ninth Circuit’s view, a plaintiff can defeat
removal by first filing a complaint that does not include
a claim or a defendant that would trigger the Act’s right of
removal and later substituting a claim or defendant that
would have triggered the right. Suppose that with the
Act’s effective date looming, the plaintiff had not com-
pleted even a minimal pre-complaint investigation. Under
the Ninth Circuit’s view, the plaintiff could sue Donald
Duck for violating a Chicago noise ordinance and then
at his leisure amend the complaint to substitute a proper
claim against a proper defendant, and the new de-
fendant would not be able to remove.
  But even the cases that reject the Ninth Circuit’s position
forbid removal if the new claim or defendant (new in the
sense of having been added after the effective date of the
Class Action Fairness Act) “relates back” to the original
claim or the original defendant. It would not do so in the
Donald Duck case, although that would not faze the
Ninth Circuit, which considers relation back important
only when necessary to avoid a statute of limitations
defense, since, if successful, the defense kills the plaintiff’s
claim rather than just forcing it to be litigated in a different
court system. McAtee v. Capital One, 

, 479 F.3d
at 1147.
  The majority view, because it makes relation back
important, raises a threshold question: should state or
federal law govern whether a claim or defendant relates
No. 08-1019                                                   5

back? The cases thus far have assumed that it is state law,
e.g., Schorsch v. Hewlett-Packard Co., 

, 750-51
(7th Cir. 2005); Plubell v. Merck & 

, 434 F.3d at
1071, but with little discussion of the issue, probably
because its resolution rarely matters (it doesn’t matter
in this case, as we shall see). What may have influenced
the assumption is that state law determines the date on
which a diversity suit was commenced for purposes of
determining whether the statute of limitations has run.
Walker v. Armco Steel Corp., 

(1980). The statute
of limitations, however, is a substantive defense the
application of which depends on the length of time that
elapsed between the date on which the plaintiff’s claim
accrued and the date on which he filed suit. If state law
governs the statute of limitations defense, as it normally
will in a diversity case, it makes sense to defer to the state
law’s determination of when a claim accrues and when a
suit is commenced, for those are the dates that determine
whether the suit is barred by the statute. But the validity of
a state-law defense is not the issue when the district court
is asked to decide in which court system, the federal or the
state, the case shall be litigated. Suppose that in an effort to
delay the effective date of the Class Action Fairness Act,
a state court defined “relation back” so broadly that even
the replacement of Donald Duck by AIGM would relate
back to the date of the original complaint and there-
fore AIGM could not remove. That would defeat the Act’s
goal, and as the Act is silent on relation back, the inter-
pretation that is consistent with that goal is permissible as
well as preferable.
  We need not choose between state and federal law (we
are merely flagging the issue for possible consideration
in the future) because Illinois’s relation-back rule is identi-
6                                              No. 08-1019

cal to the federal rule. Both rules provide that a party may
be changed if, within the deadline for service of the
complaint on it, (1) the new party had received enough
notice of the original suit that it would not be “prejudiced
in maintaining a defense on the merits” if it were brought
into the case belatedly, and (2) it “knew or should have
known that, but for a mistake concerning the identity of
the proper party, the action would have been brought
against” it. 735 ILCS 5/2-616(d)(2); Fed. R. Civ. P.
15(c)(1)(C). The plaintiff argues that these conditions
have been satisfied.
   The first may have been, but not the second. The plain-
tiff learned by December 2003, or at the latest by Novem-
ber 2004, that he had sued the wrong party, yet he
waited almost three years to substitute the right one. He
offers no excuse for having waited so long to correct
his mistake. Neither party can explain the delay.
  No matter, says the plaintiff; the rule imposes no duty
of diligence. Not in so many words; that is true. But we
must have regard for context, and in particular we
must ask why there is a relation-back doctrine in the first
place. The primary reason is to provide relief from a
defense based on the statute of limitations in cases in
which the deadline imposed by it is too tight. Just as a
statute of limitations fixes a deadline for suing, the
Class Action Fairness Act fixes a deadline for bringing
a suit that cannot be removed to federal court, and so the
relation-back issues are the same, at least when as in this
case the federal and state relation-back rules are the same.
  The purpose of allowing relation back—to extend a
deadline—allies the doctrine closely to equitable tolling,
which permits a party to file his suit after the expiration
of the limitations period if he could not reasonably be
No. 08-1019                                                   7

expected to have done so sooner. E.g., Fidelity National Title
Ins. Co. v. Howard Savings Bank, 

, 839 (7th Cir.
2006); Neverson v. Farquharson, 

, 39-40 (1st Cir.
2004). He might, for example, have known that he had
suffered a wrongful injury but have been unable to identify
the injurer in time. E.g., Singletary v. Continental Illinois
National Bank & Trust Co., 

, 1241-42 (7th Cir.
1993). But to be given the benefit of the doctrine he must be
diligent in seeking out that identity and must sue promptly
once he has learned it. E.g., Shropshear v. Corporation
Counsel, 

, 595 (7th Cir. 2001); Pacific Harbor
Capital, Inc. v. Barnett Bank, N.A., 

, 1252 (11th
Cir. 2001); Johnson v. Nyack Hospital, 

, 12-13 (2d Cir.
1996).
  It is the same here. The plaintiff thought he had been
stiffed by whoever had handled his insurance claim,
and we’ll assume that he was unable even by the exercise
of diligence to identify the handler correctly when he
sued. The relation-back doctrine would give him the time
he needed to find out whom he should have sued, and
sue him, provided that the new defendant knew about
the suit and was not prejudiced by the delay in naming
him as a defendant. But it doesn’t give the plaintiff for-
ever. It is implicit that he must proceed diligently. Other-
wise the doctrine would undercut the tolling rules.
  We have in this case a situation in which for three years
a plaintiff knows that he has sued the wrong party and
knows who the right party to sue is—for the plaintiff
does not suggest that he disbelieved AIGC when it told
him that not it but its affiliate AIGM had handled his
claim—yet he neither drops the wrong party from the case
nor adds the right one as a defendant. Throughout this
period the wrong party, though knowing it is the wrong
party, must keep tabs on the case, may have to report it
8                                              No. 08-1019

to insurance and securities regulators as a pending case,
or must incur the expense of seeking a dismissal, while
the right party wonders why it hasn’t been sued and
must make preparations for an eventual suit by lining up
counsel and preserving evidence. Neither the named nor
the mysteriously unnamed defendant may be able—
without disproportionate expense—to prove that it is
being prejudiced by the plaintiff’s dereliction, yet the
litigation process is being complicated and uncertainty
engendered and all for no reason.
   This analysis brings us back to the language of the
relation-back rules. When there is protracted and inex-
plicable delay in changing defendants, the plaintiff can no
longer argue, under either the Illinois or the federal rule,
that the defendant “should have known that, but for
a mistake concerning the identity of the proper party,
the action would have been brought against th[at] party.”
When as in this case the plaintiff does not change defen-
dants for years after discovering the mistake, the not-yet-
named defendant can no longer assume that “the action
would have been brought against” him had it not been
for the plaintiff’s mistake. After years passed without
being substituted as a defendant for AIGC, AIGM could
reasonably assume that the plaintiff had a reason for
wanting to persist in the suit against AIGC even though
that company had not handled his claim. Maybe it had
handled the claims of some members of the class, and
class counsel was thinking of substituting one of those
members for the named plaintiff and continuing with
the suit against AIGC. Maybe the plaintiff had thought
that because AIGC and AIGM are affiliated corporations
it didn’t matter which one he sued—that they could be
treated as a unit.
No. 08-1019                                                9

  What was AIGM to do? Petition the court to direct
the plaintiff to sue it? Or should AIGC have told the
plaintiff, please sue my affiliate?
   Read literally, it is true, the relation-back provision in
both the Illinois and the federal rule could be thought to
authorize relation back whenever the later-named defen-
dant should have known within the service period that had
it not been for a mistake by the plaintiff he would have
been sued, whatever he learned later. But that would be
a misreading. It is apparent that the requirement that
the party learn of the mistake before the service deadline
has passed is a limitation on rather than an expansion of
the relation-back doctrine. There can be relation back only
if the defendant realized within the period in which he
might have been sued that he should have been the
one sued. If he didn’t learn that until later, there is no
relation back because he would assume after the service
deadline passed that he was out of the woods—it would
be too late for the plaintiff to add him as a defendant.
When years passed without AIGM’s being substituted
for AIGC, whatever initial concerns AIGM might have
had about being sued would have tended to dissipate.
No longer could it be sure that the plaintiff had sued AIGC,
rather than it, by mistake. The plaintiff’s tenacious re-
tention of AIGC as a defendant suggested alternative
hypotheses to mistake, such as that, as we suggested,
class counsel intended to replace the plaintiff as the
named plaintiff in the class action suit with another
member of the class.
  We note finally that even when the conditions for rela-
tion bank are satisfied, a request to allow the complaint to
be amended is addressed to the judge’s discretion. Foman
v. Davis, 

, 182 (1962); Airborne Beepers & Video,
Inc. v. AT&T Mobility LLC, 

, 666-67 (7th Cir.
10                                                 No. 08-1019

2007); Mundt v. Ragnar Benson, Inc., 

, 16 (Ill.
1975); Brandon v. Bonell, 

, 484-85 (Ill. App.
2006); Grove v. Carle Foundation Hospital, 

, 158
(Ill. App. 2006). (Federal, but not Illinois, law permits a first
amendment to be made as a matter of right before the
answer or a dispositive motion is filed. Compare Fed. R.
Civ. P. 15(a) with 735 ILCS 5/2-616(a). That is not the
nature of the amendment at issue.) It is difficult to regard
the state court’s permitting the plaintiff to amend his
complaint to change defendants as anything other than an
abuse of discretion. The plaintiff’s delay in seeking leave to
amend was gross, unjustified, and not even explained; the
maintenance for years of a suit against a party known by
the plaintiff to be the wrong one to sue was an abuse of
legal process; and the amendment changed not only the
defendants but the claimants, since the insurance claims of
some of the class members were processed by AIGC and of
others by AIGM. If as seems likely AIGM would not have
known how substituting it in as a defendant would alter
the composition of the class, that is an independent ground
for denying relation back. Smith v. Nationwide Property &
Casualty Ins. 

, 505 F.3d at 406 n. 2.
  Obviously we cannot reverse the state judge’s grant of
leave to amend the complaint. But the arbitrariness of that
action supports an interpretation of the Class Action
Fairness Act under which the suit against AIGM is deemed
to have been commenced when it was added as a defen-
dant, and not years earlier when AIGC was sued.
  The order of the district denying the petition to remand
this suit to the state court is
                                                     AFFIRMED.

                     USCA-02-C-0072—4-22-08