You don't have to find some specific treaty or statute to say that the Bush administration has committed high crimes and misdemeanors, though it may help.

"High crimes and misdemeanors" doesn't mean a breach of some statute or written law. The term isn't limited to violations of the law for which a person can be indicted in a criminal court. The term developed at least 3 centuries before the American Revolution in English common law and for at least 600 years has meant an offense against the State, a major abuse of power, or a serious breach of the public trust by a high ranking official. The framers of the US Constitution intentionally left it undefined so that the Congress and the political process of the day could determine what constitutes a high crime or misdemeanor.

In the case of Bush, you don't have to find specific violations of a treaty or statute to impeach them. There's any number of major abuses of power and breaches of the public trust during the Bush administration to turn to, from NSA wiretapping, to rendition and torture, to the various issues involved in invading Iraq, which can provide ample grounds for impeachment.