An October/November survey by the Harvard Institute of Politics and covering the midsection (adults between 18 and 29) of the "millennial" demographic found that after the November terror attacks in France (but before the December 2 attack in San Bernardino), that demographic's support for deployment of U.S. ground troops against the Islamic State in Iraq & Syria jumped from 47 percent to 60 percent.
But when asked a followup question - "If the United States needed additional troops to combat the Islamic State, how likely would you be to serve?" - 85 percent responded "probably won't join" or "won't join."
Assuming that all or nearly all of the 40 percent who oppose a ground war answered "probably not" or "heck no," it follows that the other 45 percent who answered that way support the idea as long as it doesn't involve actually putting on uniforms, picking up rifles, and placing their own lives on the line.
I'm a war veteran myself, but I don't count myself among the "if you haven't served or won't serve, you're not entitled to an opinion" crowd. Everyone's entitled to an opinion. Even those who don't actually do the heavy lifting pick up some of the costs. They pay taxes. They support loved ones in uniform. And the risk of personal harm from blowback à la 9/11 and San Bernardino, while minimal so far, is real.
On the other hand, that differential/overlap bugs me. I wish I could get inside the heads of the three-quarters of military-age people in this survey who support the idea of a war enthusiastically enough to send others off to potentially die or return minus limbs or with traumatic brain injuries, but not enough to risk those things themselves.
As the late economist Milton Friedman pointed out, incentives change depending on whose money you're spending, and on whom. If you're spending your own money on yourself, you have a great incentive to get value for price. If you're spending other people's money on yourself, that incentive lessens - and a little more so if you're spending your money on someone else. But if you're spending other people's money on other people, the incentive pretty much disappears. Why should you care? You're neither paying the price nor gaining the benefit.
In this situation, what's being spent by those who support a war but don't plan to enlist is not money, but lives - the lives of other people (U.S. troops) to be sacrificed for other people (Iraqis and Syrians).
They're entitled to their opinions. But given the incentives, I'm not inclined to give those opinions too much weight. I'm a little older than the "millennials." I've seen this movie before. In fact, I was one of the thousands of extras. I'm against another remake.