President Barack Obama's speech on Tuesday night deserves to be cheered. Over the objections of his vice president and despite opposition from his political base, the president is sending an additional 30,000 troops to Afghanistan to fight terrorists.

But praise for Mr. Obama's decision needs to be qualified. Gen. Stanley McChrystal, the top U.S. commander in Afghanistan, had said he could use as many as 40,000 troops, a figure he arrived at after carefully evaluating what would be needed to accomplish the mission Mr. Obama...