A total of 10,000 troops will leave the war zone by the end of this year — fulfilling Obama’s promise for a withdrawal starting next month — and more than 20,000 additional forces will leave by the summer of 2012, shortly before the president will go before voters in search of a second term.

Still, almost 70,000 U.S. troops will remain in an unstable country, fighting in a war bound to see more Americans killed. Obama said they will leave at a steady pace, but the U.S. combat mission is not expected to end until December 2014 — and even then, a sizable and enduring contingent may remain in a different role.

Obama’s announcement from the White House came in a perilous political environment, with Americans soured on the war and the economy, many members of Congress pushing him to get troops home even faster, and his Republican presidential rivals taking shots at his leadership at every chance.

Plenty of disgruntled Democrats also took Obama to task, however politely, for not withdrawing more troops more quickly.

“I am glad this war is ending, but it’s ending at far too slow a pace,” said Sen. Barbara Boxer, D-Calif. Added the House Democratic leader, Nancy Pelosi of California: “We will continue to press for a better outcome.”

Costly war

At least 1,500 members of the U.S. military have died and 12,000 have been wounded since the war began in late 2001. The financial cost of the war has passed $440 billion and is on the rise, jumping to $120 billion a year. Those costs have risen in importance as a divided U.S. government struggles to contain its soaring debt.

Conceding the economic strain of waging war at a time of rising debt and fiscal constraint, Obama said it was time for America “to focus on nation building here at home.” The president’s chances for re-election rest largely on his ability to show faster job growth in a time of deepening economic pessimism.

The withdrawal is supported by the bold bottom-line claims of his security team: Afghanistan, training ground for the Sept. 11, 2001, attacks on America, is no longer a launching pad for exporting terrorism and hasn’t been for years. But that could also fuel arguments for even greater withdrawals by voters wondering what the point of the war is after all these years, especially since the face of the enemy — al-Qaida leader Osama bin Laden — was killed by American forces this spring during a raid in Pakistan.

Yet the White House insists the U.S. must maintain a strong fighting force in Afghanistan for now to keep the country from slipping back into a haven for al-Qaida terrorists.

The initial withdrawal is expected to happen in two phases, with 5,000 troops coming home this summer and an additional 5,000 by the end of the year.