In November 2009, when I visited Afghanistan, I thought I understood what our military could achieve with a troop surge.

But these days, I’m at a loss to figure out what we are doing, or what President Obama wants to achieve there. A military strategy whose chances were already slim has been undercut by his ambivalence about the war.

The White House struggle to evolve an Afghan policy — and the civilian-military tensions that emerged from it — are laid out in detail in Bob Woodward’s latest White-House-insiders-tell-all book, “Obama’s Wars.” Obama finally endorsed much of what the military asked for, sending 30,000 new troops to the conflict. But his determination to set a July 2011 date for the start of a troop drawdown undercuts the surge.

The president understands why Afghanistan still matters: If the Taliban regain control, the country will once more become a haven for Islamic terrorists. More important, a Taliban victory will rebound on neighboring Pakistan, where al-Qaida leaders are hiding and where local jihadis want the country’s nuclear bombs.

But — as Woodward’s account makes clear — Obama’s approach undercut the core of the military’s strategy: the effort to shift the momentum so that the Taliban (and most Afghans) didn’t assume their victory was inevitable.

From the military’s perspective, a shift in momentum required more than adding 30,000 U.S. soldiers or waiting for a new Afghan army to take up the struggle. It meant galvanizing Afghans from the bottom up to resist the insurgents.

In late 2009, U.S. commanders hoped to encourage tribal leaders, village elders, and district officials to stand up to the Taliban, by protecting them, providing aid funds, and enticing low- and mid-level Taliban to defect. Special Forces would help.

The central idea: If Afghans believed the Americans were committed, many would switch sides to join the likely winner, as they have done throughout their history. In 2001, many warlords abandoned the Taliban after its first big loss.

Once the momentum had shifted, it would become easier to entice some Taliban factions to accept roles in the constitutional government. And once Pakistan sensed our strong commitment, its military and intelligence agencies might stop undercutting our efforts in Afghanistan.

No question this strategy faced tremendous challenges and would have been far easier to implement in 2003 than in 2009. But it was blown off course before it was fully tested, by Obama’s announcement of the deadline.

As I saw on my last visit to Afghanistan, in April, the deadline convinced most Afghans that the Americans were headed for the exits. Tribal chiefs and village elders now believe the Taliban will make a comeback, and they are sitting tight.

Woodward details the thinking behind Obama’s decision on the deadline. “I can’t let this be a war without end, and I can’t lose the whole Democratic Party,” the president told Republican Sen. Lindsay Graham. Obama’s main focus, as spelled out in a six-page memo he made his top officials sign, is on “degrading the Taliban” to the point where U.S. forces can hand over security responsibility to the new Afghan army and police.

But there is an inherent flaw in this logic. The Afghan security forces, beset by illiteracy, attrition, and ethnic imbalances, won’t be capable of taking over until Afghanistan is already stable — and the Taliban’s momentum reversed. As I’ve noted, that reversal requires a shift in Afghan psychology at the grassroots, a belief that it is safe to turn against the insurgents. Thus, the 7/11 drawdown date practically ensures that the president’s goals can’t be met.

The question raised by Woodward’s book is why the president did not grasp this. To be fair, he inherited an Afghan mess from the Bush team that left him with only bad choices. And domestic pressures keep mounting. Yet the internal contradictions in his strategy are hard to explain.

One of Woodward’s White House sources surmised that the president felt he had to try an 18-month surge just to give the military its day and prove it could not be done. I find it difficult to believe this.

Yet it’s hard to understand what the president is up to. An Army failure in the run-up to 2012 elections will only cause greater problems for the White House. If Afghanistan implodes, it will be that much harder to start drawing down U.S. forces.

Indeed, Obama’s fortunes are now tied to the military outcome in 2011. There will be no easy way out of Afghanistan if the surge fails.

— Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer. trubin@phillynews.com