WASHINGTON • President Barack Obama rushed to the Oval Office when word arrived one night that militants with al-Qaida in Yemen had been located and that the military wanted to carry out an attack. After a quick discussion, his counterterrorism adviser, John Brennan, told him the window to strike was closing.

"I've got two minutes here," Brennan said.

"OK," the president said. "Go with this."

While Obama took three sometimes maddening months to decide to send more forces to Afghanistan, other decisions as commander in chief have come with dizzying speed, far less study and little public attention.

He is the first president in four decades with a shooting war already raging the day he took office — two, in fact, plus subsidiaries — and his education as a commander in chief with no experience in uniform has been a steep learning curve.

Along the way, he has confronted some of the biggest choices a president can make, often deferring to military advisers yet trying to shape the decisions with his own judgments — too much at times for the Pentagon, too little in the view of his liberal base. His evolution from antiwar candidate to leader of the world's most powerful military will reach a milestone on Tuesday when he delivers an Oval Office address to formally end the combat mission in Iraq while defending his troop buildup in Afghanistan.

A year and a half into his presidency, Obama appears to be a reluctant warrior. Even as he draws down troops in Iraq, he has been abundantly willing to use force to advance national interests, tripling forces in Afghanistan, authorizing secret operations in Yemen and Somalia, and escalating drone strikes in Pakistan. But advisers say he does not see himself as a war president in the way his predecessor did.

Where George W. Bush saw the conflicts in Iraq and Afghanistan as his central mission and opportunities to transform critical regions, Obama sees them as "problems that need managing," as one adviser put it, while he pursues his own mission of transforming the United States. The result, according to interviews with three dozen administration officials, military leaders and national security experts, is an uneasy balance between a president wary of endless commitment and a military worried that he is not fully invested in the cause.

"He's got a very full plate of very big issues, and I think he does not want to create the impression that he's so preoccupied with these two wars that he's not addressing the domestic issues that are uppermost in people's minds," Defense Secretary Robert Gates said in an interview.

Obama, though, has devoted enormous time and thought to finding the right approaches, Gates added. "From the first, he's been decisive and he's been willing to make big decisions," he said.

Sen. Jack Reed, D-R.I., who sometimes advises Obama, said the president was grappling with harsh reality. "He came into office with a very sound strategic vision," Reed said, "and what has happened in the intervening months is, as with every president, he is beginning to understand how difficult it is to translate a strategic vision into operational reality."

Running for president of a country at war, Obama had plenty to learn, even basics like military ceremonies and titles. Perhaps his most important tutor has been Gates, the defense secretary appointed by Bush and the first kept on by a president of another party. They are an unlikely pair, a 49-year-old Harvard-trained lawyer turned community activist and a 66-year-old veteran of Cold War spy intrigues and Republican administrations. But they are both known for unassuming discipline and bonded through weekly lunches and shared challenges.

Obama has relied on Gates as his ambassador to the military and deferred to him repeatedly.

Even on his signature campaign promise vowing to pull out of Iraq, Obama compromised in the early days of his tenure to accommodate military concerns. Instead of the 16-month withdrawal of combat forces he promised, he accepted a 19-month timetable, and he agreed to leave behind 50,000 for now rather than a smaller force.

But as he grows in the job, Obama shows more willingness to set aside Gates' advice. When Gen. Stanley McChrystal got in trouble in June for comments by him and his staff in Rolling Stone magazine, Gates favored reprimanding the commander. Obama decided instead to oust him and replace him with Gen. David Petraeus, who led the troop increase in Iraq.

"My first reaction was if McChrystal with his experience and his contacts and his knowledge were pulled out, that could have real consequence for the war," Gates said. "It never even occurred to me — I kicked myself subsequently — to move Petraeus over there. When the president raised that with me in a private meeting, it was like a light bulb went on — yes, that will work."

Obama's response to the McChrystal situation may have helped ease tension with the military.

"Ironically enough, the McChrystal firing helped a lot because Obama handled it exactly the way most senior military officers would have handled it if they had been in his shoes," said Stephen Biddle, a critic of Obama's withdrawal deadline at the Council on Foreign Relations.