The vote, set for Monday, showed how quickly the politics of President Obama’s foreign policy have shifted after he vowed in a speech to the nation to degrade and ultimately destroy ISIS.
The Arizona Republican has been hounded after a picture of him with members of the Free Syrian Army was misinterpreted and doctored purposely to make it appear that he supported ISIS, which he does not.
Many Americans agree that ISIS is a threat to the United States, but many wonder whether the country should edge toward another war.
As President Obama honored the victims of Sept. 11, 2001, the White House argued that he had the right to wage his new ISIS fight under the same legal authority he used for Al Qaeda.
Mr. Allen, who was the top American commander in Afghanistan, will coordinate the international coalition to fight the militant Islamist group, an official said.