Peace in Europe wasn't established by diplomacy alone but by military force. The President has worked hard to persuade the Russian leadership to make peace with Europe, but all his efforts have been rebuffed by Russia, often with threats of expanding and intensifying the war, so he has shifted his diplomatic efforts to threatening increased sanctions against Russia, and Russia's response is a direct threat of war against the US, and a war between Russia and the US would almost certainly be a nuclear war, so it is only prudent to reposition US forces to meet this threat. The Russians have too often repositioned their nuclear capable missiles, even in violation of existing treaties, so to have an early advantage in a nuclear war not to take their threat seriously.