Marjorie Cohn, President of the National Lawyers Guild observed:
Even though Rumsfeld didn't personally carry out the torture and mistreatment of prisoners, he authorized it. Under the doctrine of command responsibility, a commander can be liable for war crimes committed by his inferiors if he knew or should have known they would be committed and did nothing to stop or prevent them. The U.S. War Crimes Act provides for prosecution of a person who commits war crimes and prescribes life imprisonment, or even the death penalty if the victim dies.
The Pentagon has called the lawsuit "frivolous," "absurd," and "politicized." The news of the charges is just starting to circulate, but the suit is already being cited as the first step in an expected circus of witch hunt-style investigations and trials involving the Bush administration.
Whether the suit will be successful is debatable. It may never actually get to court, but it's great publicity and great copy for the news media.
The first major problem it faces is the fact there's no reason to expect the German government to follow through on a case which is substantially identical to one it dismissed in 2005 and then dismissed again on appeal, especially with a new administration which is harder on terrorism and more pro-American.
Another problem is the Germany's 2002 war crimes law's extraordinarily broad claims of jurisdiction are not recognized by the United States or even by international courts, so the inability to actually punish the subjects of the suit may get it dismissed as a waste of time and resources.
Because of these problems, the decision to go forward with the case would be inherently political and only makes sense if the German government has a desire to make a political point and a willingness to antagonize the United States. Actual prosecution of the case seems improbable, but it's still an excellent way for these activist groups to draw attention to their cause.