Pull the troops home and end the war immediately. There's no way to stop Vietnam from going communist, just let it happen. I don't care about 'weakness' or 'impeachment'. Aggressive wars don't prove 'strength', they only prove a dedication to aggressive military action which leads to violence and worse relations with the rest of the world. Real 'strength' is having the strongest economy in the world and having the largest military in the world, going to war should be a last resort and it should be defensive.
