Ambassadors from EU countries warn of retaliation for the US's limits on the visa waiver program.

Some have suggested that Europe would retaliate by imposing the same restriction on Americans, blocking Iranian-Americans etc. That would be a foolish way to retaliate, since the Americans who support the recent restriction would be glad to see Iranian-Americans punished. As retaliation, it would be ineffective.

My suggestion is that Europe should require visas for Americans whose names begin with "R", or perhaps "Ry". That might be a small fraction comparable to the fraction of Europeans affected by the new US restrictions.

Another idea: require visas of Americans related to Republican congresscritters.