An upcoming U.S.-led military campaign to regain control of the Taliban heartland of Kandahar will be a decisive phase in the Afghan war, Defense Secretary Robert Gates told frontline troops this week.

The United States is deploying 30,000 extra troops to Afghanistan this year in an effort to turn the tide against a growing Taliban insurgency. Washington plans to start withdrawing by mid-2011, making this year a critical period for the outcome of the eight-year-old war.

Following are answers to some questions about the U.S. surge and the military strategy this year.

WHAT IS THE OBAMA SURGE?

By the last months of this year, the United States will have about 100,000 troops in Afghanistan, three times as many as when President Barack Obama took office in January, 2009.

Some of those extra troops were sent by departing President George W. Bush; 22,000 were sent by Obama in his first months in office. Last August, the U.S. and NATO commander, General Stanley McChrystal, asked for more.

Obama agreed in December to send the remaining 30,000. A quarter of the 30,000 extra troops have so far arrived and took part in an assault on the town of Marjah that began last month, billed as the biggest offensive of the war.

Most of the rest of the combat power will be deployed to Kandahar, the Taliban's former spiritual homeland, in coming months, for an even bigger operation to assert control of Afghanistan's second largest city and surrounding areas.

Others are coming to speed up the training of Afghan security forces, so that they can take over and Western troops withdraw.

McChrystal's campaign plan calls for the extra troops to be used quickly to have a decisive effect in turning the momentum in the south by the middle of this year. After that, the focus will switch to consolidating gains and improving Afghan government institutions until troops begin gradually to withdraw in 2011.

Other NATO allies have pledged to add to their contingents, which now total more than 40,000 troops. But two of the biggest, the Netherlands and Canada, have announced plans to withdraw their combined 5,000 troops in 2010 and 2011.

WHAT ELSE NEEDS TO BE DONE SO WASHINGTON CAN WITHDRAW?

NATO has set an ambitious target to increase the size of the Afghan security forces by nearly 50 percent to 305,000 by October 2011, to create a force big and capable enough to start looking after the country's security.

McChrystal has said he would eventually like to see that target raised to 400,000, although it is not clear if funding is available to do that.

The Obama administration has ordered a civilian surge of hundreds more advisers, diplomats and aid workers in a bid to strengthen Afghan institutions and provide support for the population, particularly in areas where troops move in.

The track record of the aid effort since 2001 has been spotty at best, with billions spent, much lost to corruption or security expenses, and many Afghans complaining of limited improvements to ruined infrastructure and scant progress fighting poverty.

The plan also depends on boosting the political credibility of Karzai's government, seen at home and abroad as corrupt and ineffective. A parliamentary election will be held in September, and Western governments accept it will be difficult to guard against a repeat of massive fraud that occurred when Karzai was re-elected president last year.

WHAT ABOUT PEACE TALKS?

Karzai has stressed efforts both to persuade lower-level Taliban fighters to lay down arms and, more controversially, to negotiate with insurgent leaders.

He is planning to hold a peace conference with tribal leaders in Kabul in late April, and received backing from Western countries for the plans at a conference in London in January. But so far any talks are still just an idea.

Optimists say Western backing for Karzai's offer -- and signs of support from Pakistan -- could lure the Taliban to the table. But pessimists say the fighters are unlikely to make many concessions as long as they think they are winning. With Washington planning to start withdrawing next year anyway, they may believe they need only to wait out the clock.

U.S. officials are guarded on the subject of talks. Gates expressed support for Karzai's efforts this week, but also said it was too early to see whether military pressure from the surge would push fighters to the negotiating table, because most of the new troops have yet to arrive.

Some U.S. officials would prefer to wait until NATO forces are perceived as having the military advantage before high-level talks start with Taliban leaders.

WHAT ARE THE POLITICAL RISKS FOR OBAMA?

Polls show the U.S. public is split over the war, but support has ebbed as costs and casualties rise. Many in Obama's own Democratic Party are uneasy about the war, and some lawmakers openly oppose it.

On March 10, the 435-seat House of Representatives overwhelmingly rejected a resolution to pull out of Afghanistan. But 65 lawmakers, most of them Democrats, supported the resolution, showing the degree to which liberal Democrats are unhappy about the president's policy.

There are mid-term congressional elections in November and it will be important for Democrats to show constituents that Obama's new strategy has shown some progress and that troops are closer to coming home.

Obama himself faces re-election in 2012. By that time -- unless plans change again -- the withdrawal should be well under way. Obama will have to make the case that U.S. troops will leave behind an Afghanistan better able to look after its own security, and prevent a return of groups like al Qaeda responsible for the September 11 attacks that triggered the war back in 2001.