US law generally doesn't allow actions to be made illegal after the fact (ex post facto). So no, the act must be illegal when it was committed.

In this case, though, copyright infringement is already in principle illegal whether a work has been registered or not; the registration requirement is just a procedural requirement that bars collecting damages, not something that makes the action legal or illegal.

However U.S. copyright law does have a statutory limit on how retroactive registrations can be. If you file a registration within 90 days of first publication, the registration is retroactive to the date of publication; but if you file more than 90 days from publication, registration is only effective from the date of registration.

Fwiw, even with unregistered works you have some options available to you, just not damages. For example, you can file a DMCA takedown notice.

If you file a DMCA notice and the infringing party files a DMCA counter-notice, would you have to register the copyright before you could sue for infringement?

I believe the answer is yes, you need to first register before filing suit. If the offending work continues to be published, you can register and then sue.

An exception is foreign works, whose owners can file a suit without first registering, since treaty obligations don't allow the U.S. to subject foreign holders to "formalities". However foreign holders are limited to recovering "actual damages" in such suits, and need to register to be eligible for per-copy statutory damages.

You're supposed to register within 5 years, but if you put it off it's possible that an infringing party could register before you. I can't think of any cases where this actually happened, though.

not quite.

In general, copyright registration is a legal formality intended to make a public record of the basic facts of a particular copy­right. However, registration is not a condition of copyright protection. Even though registration is not a requirement for protection, the copyright law provides several inducements or advantages to encourage copyright owners to make regis­tration. Among these advantages are the following:

• Registration establishes a public record of the copyright claim.

• Before an infringement suit may be filed in court, regis­tration is necessary for works of U.S. origin.

• If made before or within five years of publication, regis­tration will establish prima facie evidence in court of the validity of the copyright and of the facts stated in the certificate.

• If registration is made within three months after publica­tion of the work or prior to an infringement of the work, statutory damages and attorney’s fees will be available to the copyright owner in court actions. Otherwise, only an award of actual damages and profits is available to the copyright owner.

• Registration allows the owner of the copyright to record the registration with the U.S. Customs Service for pro­tection against the importation of infringing copies. For additional information, go to the U.S. Customs and Border Protection website at www.cbp.gov/.

Registration may be made at any time within the life of the copyright. Unlike the law before 1978, when a work has been registered in unpublished form, it is not necessary to make another registration when the work becomes published, although the copyright owner may register the published edition, if desired.

Wait, so correct me if I'm misunderstanding here, but these two things:

  >However, registration is not a condition of copyright protection.

  >Before an infringement suit may be filed in court, regis­tration is necessary for works of U.S. origin.

..seem mutually exclusive. If you have copyright protection without registering, but can't sue without registering, what's the point of the pre-registration protection?

You can register after infringement but before suing. It is sort of like saying murder is illegal, but a suspect has to be arrested before he can be tried.