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The United States already deploys about 68,000 troops, and there are an additional 20,000 or so of various stripes from friendly countries. General Stanley McChrystal, NATO’s commander in Afghanistan, asked for about 44,000 additional American troops on the ground. The 30,000-plus that Mr. Obama will approve is about the maximum that could actually be deployed to Afghanistan in the next 12 months. And if the general can clearly demonstrate he still needs more in a year, the president will signal his willingness to consider an additional 10,000. But that would be the limit, the end of it.

The strategy to govern the employment of these forces, Mr. Obama is expected to say, will be much like the counterinsurgency approach he originally approved back in March—the approach...