How can a crime be considered a crime?

What you are referring to is called an 'ex post facto' law, wherein a new law is passed that makes a previous action illegal.

Consider this passage (from Wikipedia):

"In the United States, the federal government is prohibited from passing ex post facto laws by Article I, section 9 of the U.S. Constitution and the states are prohibited from the same by clause 1 of section 10. This is one of the very few restrictions that the United States Constitution made to both the power of the federal and state governments prior to amendment. Over the years, when deciding ex post facto cases, the United States Supreme Court has referred repeatedly to its ruling in the Calder v. Bull case of 1798, in which Justice Chase established four categories of unconstitutional ex post facto laws. The case dealt with Article I, section 10, since it dealt with a Connecticut state law.

However, not all laws with ex post facto effects have been found to be unconstitutional. One current U.S. law that has an ex post facto effect is the Adam Walsh Child Protection and Safety Act of 2006. This law, which imposes new registration requirements on convicted sex offenders, gives the United States Attorney General the authority to apply the law retroactively.[2] The U.S. Supreme Court ruled in Smith v. Doe (2003) that forcing sex offenders to register their whereabouts at regular intervals and the posting of personal information about them on the Internet does not violate the constitutional prohibition against ex post facto laws, because compulsory registration of offenders who completed their sentences before new laws requiring compliance went into effect does not constitute a punishment.[3]"

It is unlikely that the Constitutionality of any attempt to prosecute individuals under an ex post facto law covering waterboarding would be upheld. However, as I understand it, the true danger is that there is an attempt to substantiate the position that it was *never* legal in the first place; if such a thing could be proven (and upheld), we would no longer be talking about 'ex post facto' at all.

Torture WAS wrong then as well. What you are missing is the idea. The idea is to see IF the laws were stretched, or if there was enough wiggle room that the torture was somehow legal. Remember the lawyers for the Bush Administration outlined how it was legal, this isn't an attack on former President Bush so far, but on the lawyers that wrote the memos saying torture was legal.

Unless the law in question is retroactive, (ex post facto,) a person cannot be prosecuted for a crime they committed before that law was enacted.

Okay I'm like really confused about your question. But what I know about our law is that you cannot be prosecuted for something that we do not have laws for yet.

So if you do something wrong but its technically not illegal then nothing can me done.