If they don't, they risk prison. In the military you don't pick and choose which orders to obey and which promises to keep.
But what can those who serve their country do, when government breaks its promises to them?
Not much apparently. But that's what happened this week when 4,000 of our soldiers were told they would have to stay in Iraq longer than the government had promised.
It has happened over and over. Combat tours have been extended, soldiers have been kept on active duty longer than their term of enlistment, reserves who thought they had completed their combat obligations have been called back.
Soldiers told they would get two years at home between combat assignments are going to Iraq every other year — some for the third time.
The Pentagon claims it has every legal right to do this and it may well be legal. But it is not right break our promises to the very people who are bearing the brunt of this war.
Our army is just too small to carry out the mission it has been assigned. We need to fix that or change the mission and find a way out of this war.
With an all volunteer force we have no choice. If Americans come to believe the Pentagon can't be trusted to keep its promises to those who join, no one will volunteer.
Then where will we be?
Bob Schieffer is broadcast journalism's most experienced Washington reporter. He is CBS News' Chief Washington Correspondent and also serves as anchor and moderator of Face The Nation, CBS News' Sunday public affairs broadcast.
Schieffer served as interim anchor of The CBS Evening News from March 10, 2005 until Aug. 31, 2006. He will be a regular contributor to The CBS Evening News with Katie Couric.