ROME — President Trump made the most of his short time in Italy. He was treated to a private audience with the pope, met with both the country’s president and its prime minister in Rome, flew to Sicily for a summit meeting of world leaders and visited with American troops at a nearby naval air station.

But as the sudden burst of diplomatic activity subsided with his departure, European and American officials fear a return to the new normal of American inattention as the administration struggles with political turmoil and Russia-related scandals back home.

All the while, Russia is assiduously courting Italy, a country that once had the largest Communist party outside the Soviet bloc and that many analysts consider the soft underbelly of the European Union.

In Rome, Mr. Trump left behind an embassy without an ambassador, and forfeited a geopolitical playing field that Moscow’s ambassador in Rome, Sergey Razov, is exploiting.