WASHINGTON — President Obama’s decision on Wednesday to radically shift United States policy toward Cuba is the latest and most striking example of a president unleashed from the hesitancy that characterized much of his first six years in office.

The announcement, made in a speech to the nation from the Cabinet Room of the White House, follows similar decisions by Mr. Obama in recent weeks to defy Republicans on immigration, climate change policy, the regulation of the Internet and negotiations with Iran.

Gone are the cautious political calculations that consigned contentious issues to secondary status and led some of the president’s strongest allies to accuse him of abandoning his principles. Mr. Obama is now pushing forward aggressively on his promised agenda and ignoring his most ardent critics.

“He’s going down a checklist of thorny, longstanding problems, and he’s doing whatever he can to tackle them,” said David Axelrod, a former senior adviser. “These are things that have been tearing at us for decades and generations. My sense is his feeling is, ‘I’m not going to leave office without doing everything I can to stop them.’ ”