Article 26 of the German constitution (Grundgesetz) is the one article, whose interpretation could become quite interesting when the US finally start their war against Iraq. It prohibits wars of aggressions and their preparation. In detail it says (translation from http://www.lib.byu.edu/~rdh/eurodocs/germ/ggeng.html):

Activities tending and undertaken with the intent to disturb peaceful relations between nations, especially to prepare for aggressive war, are unconstitutional. They shall be made a punishable offense.

(The following translation graciously provided by eliserh seems to be better although less official: Acts which are capable of and taken with intent to disturb the peaceful coexistence of nations, including without limitation the preparation of a war of agression, are unconstitutional. Such acts shall be subject to criminal penalties.)

After World War II, when the Germans made their constitution, this article was probably loudly applauded by the Americans as well as Germany's neighboring countries. Now, once Germans have become a bit more pacifistic than 60 years ago, the article might be seen in a different light. It could be the basis to arrest US (or other European, e.g. British) soldiers stationed in Germany which are planning or moving to participate in a war against Iraq not consented by the UN. Lets have a look at such a scenario.

First: Would a war against Iraq be an aggressive war?

It seems to be (but IANAL) that most German specialists for constitutional law agree that a war is not aggressive if:

  1. Germany is attacked and defends itself. Due to article 24(2) this also holds if any member of the NATO is attacked.
    This is the reason why Germany's participation in the war against Afghanistan was legal (though probably a shady "legal"). The US were attacked by irregular troops supported by Afghanistan's government. Germany helped to defend.
  2. The UN mandates the actions.
    Otherwise Germany couldn't partake in many UN missions without lots of discussions.
  3. The war is fought to defend basic human rights (like survival) against genocide and similar atrocities.
    The most controversial reason since it does away with the previously well-established principle of non-interference into internal affairs of other countries. This reason was used as justification of the NATO intervention in Kosovo.

Now let's look at a possible war against Iraq without a specific UN mandate. Reason 1 doesn't hold. Reason 3 currently also doesn't hold. Saddam did kill his own people but that happened many years ago. The close call is the second reason. Depending on whom you listen to, the "severe consequences" threatened on Saddam by the UN either already sanction an attack or not. Therefore the ultimate decision if an Iraq war falls into the category of forbidden wars of aggression would probably have to be made by the German Supreme Court.

Second: Could foreign soldiers be punished?

Article 26 prohibits any preparation within the reach of the German constitution, so one could assume that German police would start storming American and British bases in Germany if the US and Great Britain declared such a war. But the article hasn't been transferred faithfully into penal law. Article 80 of the German penal law (Strafgesetzbuch) is this transfer and it reads (my translation):

Preparation of a war of aggression. Any person that is preparing a war of aggression (article 26 sec. 1 basic law), in which the Federal Republic of Germany is supposed to participate, and thereby imposes the risk of war onto the Federal Republic of Germany shall be punished with a lifelong sentence or with prison for not less than 10 years.

(BTW, Article 80a StGB prohibits to goad people into a war of aggression, so don't start handing out "Attack Iraq now!" flyers during your next holiday in Germany. ;-)

What this seemingly means is that you can do anything within German borders as long as Germany is not involved. So actually the US should be thankful for Mr. Schroeder's unwillingness to commit any German help to a war against Iraq. As long as Germany does not partake in the war, US soldiers in Germany are safe! (As I said, IANAL. So don't feel too safe. )
Of course, if German politicians can be swayed by American pressure, things will become a lot more interesting.