Six months after it announced its strategy for Afghanistan, the Obama administration is sending mixed signals about its objectives there and how many troops are needed to achieve them. The conflicting messages are drawing increasing ire from U.S. commanders in Afghanistan and frustrating military leaders, who are trying to figure out how to demonstrate that they're making progress in the 12-18 months that the administration has given them. Adding to the frustration, according to officials in Kabul and Washington, are White House and Pentagon directives made over the last six weeks that Army Gen. Stanley McChrystal, the top U.S. military...