WASHINGTON — The relative caution that constrained President Trump for much of his first year in office has been cast aside, and an emboldened commander in chief is finally reshaping foreign policy to reflect the “America First” philosophy he promised during his campaign.

Having shed or sidelined some of the top advisers who held him back in the past, Mr. Trump gives the appearance of a leader liberated at last to follow the china-breaking instincts that have long animated his approach to the world even as they troubled diplomats and national security veterans of both parties.

The president’s decision to pull out of the Iran nuclear deal on Tuesday may be only the start of a period of several weeks in which he repositions the United States in the world in a way that could last for years. After breaking with European allies over the Iran agreement, Mr. Trump will break with Arab allies on Monday with the formal opening of an American Embassy in Jerusalem.

He has until the end of the month to decide whether to impose punishing steel tariffs on key American trading partners. He has said he hopes to forge a new trade deal with Mexico and Canada within weeks or blow up the North American Free Trade Agreement. Then he will test his theory that he can force the mercurial North Korea to surrender its nuclear arsenal through “maximum pressure” coupled with threats of military action followed by high-stakes one-on-one diplomacy.