WASHINGTON — Chuck Hagel, President Obama’s nominee to be secretary of defense, on Thursday morning said that the United States must lead other nations in confronting threats, use all tools of American power in protecting its people and “maintain the strongest military in the world.”

In an opening statement at his Senate confirmation hearing, Mr. Hagel presented a broad, forceful endorsement of American military power aimed at answering critics who say he would weaken the United States. He offered strong support for Israel, said he was fully committed to the president’s goal of preventing Iran from obtaining nuclear weapons and said he would keep up pressure — through Special Operations forces and drones — on terrorist groups in Yemen, Somalia and North Africa.

“I believe, and always have, that America must engage — not retreat — in the world,” Mr. Hagel told members of the Senate Armed Services Committee.

On Afghanistan, which Mr. Hagel called “the longest war, as we all know, in America’s history,” Mr. Hagel, a Republican and former senator from Nebraska, said he agreed with the president that there would only be two functions for the small number of American forces in Afghanistan after 2014: hunting down Al Qaeda and its affiliates and training and advising Afghan security forces.

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