The recent death of U.S. citizen Douglas McAuthur McCain while fighting for the Islamic State and the reported presence of more than 100 Americans in similar roles on the battlefield in Iraq and Syria raise prickly issues for the U.S. government and the public.

The first concern is the normal one for parents and friends of these Americans. They face a loss, from the person’s absence or even death. They also may be perplexed why their loved one abandoned American society to fight alongside a foreign organization opposed to American values, if not to the United States itself.

The phenomenon of Americans enlisting as soldiers in foreign causes, both good and bad, is not new. Americans went off to fight in World War I before the United States entered that conflict in 1917. The famous Abraham Lincoln Brigade numbered 2,800 and fought against the fascists on the side of Republican Spain in the 1930s. Hundreds of American citizens are enlisted in the Israeli Defense Forces at any given time. Two Americans died in the recent Gaza war.

It is, in principle, within the powers of the U.S. government to prohibit such activities, but it is not easy to do so without infringing on the fighters’ personal liberties. Borders in these troubled areas are not sharply enforced in any case.

The most serious worry of loyal Americans over fellow citizens who are engaged in combat in the Middle East is that they subscribe to ideas and gain military skills that may be used against the United States and in favor of the organizations they supported overseas.

Spying on them more over there or back here may help to a degree, but it may just as likely intensify their alienation and stealth in planning, than neutralize them when they come home. It is not an easy problem to solve.