When President Bush said last week that June had been a ''tough month'' in Afghanistan, where more US and NATO troops died during the past two months than in Iraq, he was pointing to a growing problem in this country's military commitments overseas. The problem consists of having too many soldiers pinned down in one place -- Iraq -- and not enough devoted to fighting an increasingly difficult struggle in another -- Afghanistan. It's unlikely that Mr. Bush can do anything in his remaining months in office to keep the problem from getting worse.