WASHINGTON — President Obama’s decisions this week to retain important elements of the Bush-era system for trying terrorism suspects and to block the release of pictures showing abuse of American-held prisoners abroad are the most graphic examples yet of how he has backtracked, in substantial if often nuanced ways, from the approach to national security that he preached as a candidate, and even from his first days in the Oval Office.

Mr. Obama’s opening gambits as president were bold declarations of new directions, from announcing the closing of the detention center at Guantánamo Bay, Cuba, to sweeping restrictions on interrogation techniques. He advertised both as a return to traditional American values, after the diversions taken by George W. Bush to the detriment of America’s image abroad and of itself.

But as he showed this week in the way he dealt with those two hard cases, Mr. Obama has begun to scale back. Faced with the choice of signaling an unambiguous break with the policies of the Bush era, or maintaining some continuity with its practices, the president has begun to come down on the side of taking fewer risks with security, even though he is clearly angering the liberal elements of his political base.

Mr. Obama balked on releasing the photographs of prisoners after the military — and his influential defense secretary, Robert M. Gates, the cabinet’s one holdover from the Bush administration — argued that making them public would hand Islamic militants a propaganda coup that could lead to renewed attacks on American forces.

In announcing on Friday that he would retain the military commission system set up by Mr. Bush, even while expanding the rights of detainees to mount a vigorous defense, Mr. Obama suggested that there was no inherent conflict between keeping the nation safe and reasserting values that he and many of his supporters believed had been swept aside during the Bush years.

“This is the best way to protect our country, while upholding our deeply held values,” Mr. Obama said in a statement.

The issues of prisoner abuse and military commissions are hardly the only areas where clean breaks with the past have proven more problematic than expected. In ordering a buildup of troops in Afghanistan, Mr. Obama has taken steps Mr. Bush hesitated to take, partly out of denial that a war America had been winning in 2002 had turned so bad.

In authorizing continued covert action in Pakistan and agreeing to a slower pace of withdrawal from Iraq than he talked about during the campaign, he has raised the question of whether new facts, more hawkish advisers or the drumbeat of daily threat assessments have subtly changed his willingness to throw the gearboxes in reverse. On both the left and the right, there is speculation about whether the influence of Mr. Gates, or of his national security adviser, Gen. James L. Jones, or of Secretary Hillary Rodham Clinton, had forced him to revise his view.

Perhaps it is the knowledge that lives rest on his choices, or the general sense that settles over most presidents that the world is a more complex place from the vantage point of the Oval Office than it appears from the campaign trail.

And there is an undertone of political realities at work. Mr. Obama would have had to shut off every television set in the West Wing in recent days to avoid seeing former Vice President Dick Cheney’s assaults on his national security policies. While the Cheney fusillade has left many Republicans wincing, it has reminded Democrats that they could be politically vulnerable should the United States be attacked again.

Pete Wehner, a member of Karl Rove’s staff in the Bush White House, who applauded several of Mr. Obama’s decisions this week, argued in an interview that “this is about more than simply discovering, once you are president, that the world is a complicated place.”

“It’s a reminder of how quickly we lose the real nuances of governing when you are campaigning,” Mr. Wehner said. “For many on the left who supported him, this is just this side of a betrayal, but for those who argue that he is a pragmatist, national security is the arena where they can most effectively make that case.”

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