BARCELONA — Last month, as Europe finally lifted COVID-19 restrictions, the mood was jubilant across the continent. Outdoor mask mandates and curfews were dropped, Americans were cleared to resume travel to tourist mainstays, and hopes rose that life would quickly return to normal. The swift spread of the Delta variant, however, has upended all of that wishful thinking and is offering a warning to the U.S. Fast becoming the dominant strain of COVID-19 across Europe, Delta is wreaking havoc in Spain, Portugal and the United Kingdom. Spain alone reported nearly 44,000 cases on Tuesday, doubling the number recorded one week...