WASHINGTON — The United States will withdraw all of its troops from Iraq by the end of this year, officially ending the long, divisive war that began in March 2003, President Barack Obama announced Friday.

“After nearly nine years, America’s war in Iraq will be over,” Obama said in the White House briefing room. The last U.S. soldiers will leave by Dec. 31, Obama said, “with their heads held high, proud of their success, and knowing that the American people stand united in our support for our troops. That is how America’s military efforts in Iraq will end.”

Obama cast the announcement as fulfillment of his 2008 campaign pledge to end the war in Iraq. It has cost the lives of more than 4,400 Americans and tens of thousands of Iraqis, with more than 30,000 U.S. troops wounded. It has also cost U.S. taxpayers around $800 billion so far.

Obama emphasized that he’s also winding down the U.S. military presence in Afghanistan, which he said is becoming more stable. “The tide of war is receding,” Obama said. “Even as we remove our last troops from Iraq, we’re beginning to bring our troops home from Afghanistan.”

Framing the announcement that way is likely to help Obama politically, especially with his somewhat disenchanted Democratic base, as he begins his re-election campaign. But in fact, complete U.S. withdrawal from Iraq was not what U.S. military leaders wanted, nor is it what Obama had been seeking.

Rather, both the Pentagon and Obama wanted to station a residual U.S. military force in Iraq indefinitely to help stabilize the troubled nation, train its troops — and deter neighboring Iran from meddling.

But Iraq’s government refused to grant any remaining U.S. troops immunity from prosecution for crimes, a stand the Pentagon considered a deal-breaker. So Obama chose complete withdrawal, even though analysts believe it poses some risk to U.S. strategic interests.

Most Democrats lauded Obama’s decision, while some Republicans were critical, saying a troop exit could jeopardize any gains made in the country.

“I hope I am wrong and the president is right, but I fear this decision has set in motion events that will come back to haunt our country,” said Sen. Lindsey Graham, R-S.C., a leading congressional voice on military affairs.

And Sen. John McCain of Arizona, the top Republican on the Armed Services Committee, warned on CNN that “there is a renewed risk of greater violence and renewed Iran penetration in southern Iraq, and a possible return to sectarian conflict.”

Republican presidential candidate Mitt Romney also had harsh words for Obama, saying his “astonishing failure to secure an orderly transition in Iraq has unnecessarily put at risk the victories that were won through the blood and sacrifice of thousands of American men and women.”

He questioned whether the decision was “the result of a naked political calculation or simply sheer ineptitude in negotiations with the Iraqi government.”

However, House Speaker John Boehner, R-Ohio, said that while he was concerned that withdrawal could jeopardize gains, “I’m hopeful that both countries will work together to guarantee that a free and democratic Iraq remains a strong and stable partner for the United States in the Middle East.”

Rep. Jerrold Nadler, D-N.Y., a leading member of the Congressional Out of Iraq Caucus, said that the U.S. “never should have invaded Iraq in the first place.” But he applauded Obama for “following through on his promise to end the conflict.”

Yet Rep. Dennis Kucinich, D-Ohio, a longtime war critic, said the U.S. “fails to acknowledge that we will simply be replacing one U.S. occupation with another.”

He said the State Department would still maintain a “massive presence” in Iraq, along with “heavily armed private security contractors,” whom he argued would “continue to foment instability and violence in Iraq and the region.”

“We need to get out now, not just trade uniforms and personnel,” Kucinich said. “It is reasonable to ask whether the people of Iraq will notice any change.”