There's a law, called USERRA (Uniformed Services Employment and Reemployment Rights Act), that says their employers have to take them back at the same pay.
But what 60 Minutes correspondent Lesley Stahl found is that despite the law, thousands of guards and reservists come home to find themselves demoted or penalized, or out of a job completely.
Army Reservist Joanne Merritt is a nurse who works with wounded soldiers. When she got back to her regular job in 2006 after a two-year deployment, she was told her job was gone since she had been away too long.
"We had to give that to somebody else. So, your job is no more," she remembers.
Despite the law, she couldn't have her job back. "Yes. It really hurt. It hurt because I just wasn't expectin' this."
Considering who her employer is, why would she expect it: Merritt works at, of all places, the VA Medical Center in Augusta, Ga.
Asked if the Veterans Administration did not understand this law, Merritt says, "Yes. And I said, 'You know what? I am not going to accept this.'"
Merritt filed a complaint against the VA, and within a couple of weeks got her old job back, plus the back pay and leave that she was entitled to, which the VA had also denied her.
Asked if she has any sympathy for the VA though, considering she was gone two years, Merritt tells Stahl, "I've been in the military for 25 years now. At that point, it was 23. The president writes an order, gives an order, I follow it. What excludes the VA? No, ma'am, I do not have any sympathy. I feel that the laws are there for reasons."
According to the Pentagon, over 10 percent of guardsmen and reservists report having problems when they return to work; tens of thousands have filed complaints or lawsuits against their employers.
Assistant Secretary of Defense Thomas Hall is the top man at the Pentagon overseeing the Reserves and National Guard. He says he is aware that there are reservists and guardsmen working for the Department of Defense and in the Pentagon who have come home and also encountered problems.
How does he explain that?
"Well, we want the government to be the model employer…," he says. "What I'm saying is that the federal government's entire leadership has committed that they will be. Do we need to do better? Yes."
The federal government is one of the largest employers of guardsmen and reservists, but they also work at over 100,000 private companies. Lawsuits under USERRA have been filed against some of the largest companies, like Wal-Mart, American Airlines, and UPS.
It's in that private sector where 60 Minutes discovered that a rebellion is brewing.
"The private employers cannot supplement, cannot support the full cost of defending this nation on our balance books," says Dave Miller, vice president of Con-way freight, a national trucking firm.
After 9/11, he saw it as a patriotic duty to back his guard and reserve employees 1,000 percent.
But now his patience is wearing thin. Not only are the deployments long, his drivers, mechanics and others are being called up for a second and third tour of duty, often on such short notice it's hard to find a replacement.
He says they typically get about three weeks' notice - not a lot of time to run an ad, and bring people in for an interview. And he says they never know how long the deployment is going to be.
With over 50 of his workers deployed right now, Miller says the company is spending about $4,000 apiece to train their replacements, and as much as $100,000 if a worker has to be relocated from another state.
Take the case of one of his drivers, Jeff Vineyard. In 2005, Vineyard was sent to Iraq for a year as a member of the Indiana National Guard, where he says he was driving jet fuel.