Americans will be required to get an ‘ETIAS’ (European Travel Information and Authorization System) in order to travel to Europe, which is essentially what the US requires of Europeans (an ‘ESTA’). It’s advance permission to travel.
Of course the game of tit-for-tat is far from over. Two years ago the US Secretary of Homeland Security said he wanted to revisit allowing Europeans to travel to the US with ‘only’ an ESTA. By the way the number of people killed by terrorists who entered the U.S. under the Visa Waiver Program is zero.