For a fleeting period this summer, Europe nearly resembled its old borderless self, with people zipping across the continent, unburdened by restrictions or mandatory quarantines, as they vacationed along the French Riviera or on the Greek islands. But Europe’s travel free-for-all lasted just a matter of weeks.

With coronavirus cases rising after an early-summer ebb, governments across the continent are abruptly rethinking the wisdom of an open Europe, while reinstituting quarantines and other border controls. The changes reflect a sense that travel — and the attempt to reboot the Mediterranean’s tourism economy — has undercut Europe’s fight to control the virus.

Vacation-popular Greece and Croatia, which largely missed Europe’s first wave, have seen cases surge in some of their most visited regions and are now dealing with their largest outbreaks to date.

European governments have agreed that it’s okay to curtail free movement — a fundamental principle of the European Union — from places that have higher rates of transmission. But the newly imposed restrictions can be complicated to follow.In some cases, they apply to everyone traveling — or returning — from a specific country. In other cases, they are limited to certain “high risk” areas. People traveling from Barcelona to Belgium must observe a mandatory 14-day quarantine. For those who have been in Madrid, a quarantine is merely recommended.

Britain, which is negotiating its post-Brexit relationship with the European Union, has been steadily removing European countries from its safe list. First Spain and Luxembourg in late July. Then Belgium, France, Malta and the Netherlands. The removal of Croatia and Austria was announced this week, effective Saturday — a decision that forced tens of thousands of vacationers to either race home or be subject to quarantine.

There was similar chaos when European leaders shut down borders to control the coronavirus in March. But by mid-June, with cases steadily declining, they agreed to remove most restrictions within the region.

A surprisingly robust pandemic vacation season followed, even with travelers from the United States banned. That was an especially welcome relief for the Mediterranean countries that are heavily reliant on tourism. Croatia, for example, reported earlier this month that it was hosting more than 800,000 tourists, 70 percent of the total from a year ago.The return of foreign tourists from vacation in Zagreb, Croatia, resulted in a backup of cars at the Lucko toll station on Aug. 15. (Denis Lovrovic/AFP/Getty Images)

But just as Europe’s travel season is reaching its peak, officials in country after country have started worrying about a second wave. Spain, in particular, has seen case numbers spike. France and Germany are now contending with their highest infection rates since April.