After weeks dominated by health care, President Barack Obama's secret trip to Afghanistan turned attention back to another issue whose progress this year could help define the success of his presidency.  By deciding in December to order a massive buildup to the war he inherited, Obama placed a big bet.  Nearly tripling the U.S. presence with 30,000 more troops, he escalated an unpopular war that has seen few gains in its eight years.  Those new forces are still flowing in, and the first major campaign under Obama's new strategy was launched last month, in the south.  Although Afghanistan has been eclipsed recently by the contentious, cliffhanger health care debate, Obama's daunting challenge has not gone away.  O b a m a m u s t s h o w Americans that the big infusion of U.S. troops, to reach a total of about 100,000 by summer, will be worth the additional loss of life.  As Obama traveled to Afghanistan on Sunday for his first visit to the war zone as president, his aides made clear they understand the challenge and the importance of this year.  "This is really a strategic moment in the history of our involvement," national security adviser Jim Jones told reporters aboard Air Force One during the covert overnight flight to Afghanistan.  Whether he was talking to Afghan President Hamid Karzai in the presidential palace in Kabul or before 2,500 cheering American troops at Bagram Air Field about 50 miles away, the message during his six hours on the ground was the same: Afghan leaders, particularly Karzai, must step up now and make progress on old demands.  Those include reducing corruption, ensuring the delivery of basic services to Afghans, providing true rule of law with an effective judicial system, turning away warlords and unqualified cronies from government positions and creating an effective national police force and army.