SMEDEREVO, Serbia — When President Xi Jinping of China chose an industrial town on the Danube River to announce that Serbia was at the center of a $900 billion “One Belt, One Road” infrastructure initiative, it was a bold declaration that his country had set up shop on Europe’s southeastern flank.

Standing on the grounds of a Communist-era steel plant here in Smederevo, about 28 miles east of Belgrade, the capital, Mr. Xi promised last year to pour money into roads and railways to create a transport corridor for Chinese goods to flow to West European markets.

Called the New Silk Road, the route would run from China to Germany, via the port of Piraeus in Greece, passing through the Balkans.

Mr. Xi’s decision to extol his signature policy in Serbia, one of the poorest countries in Europe, was a canny move that put the initiative on a collision course with the European Union’s projects in the region.