ROME — Italy has given Europe many things, but rarely leadership.

Among member states of the European Union, Italy is important but not always influential, partly because of the decades-old dysfunction of its politics. France and Germany traditionally set the European agenda, while Italy is often a junior partner, if sometimes a comic sideshow.

But with the European Union fragmenting politically and hit by crises, Prime Minister Matteo Renzi is arguing that Italy’s voice must now be heard and be taken seriously. He has taken a confrontational approach, partly out of frustration, which has brought new tensions in the bloc, even as all sides have sought to tamp down the conflicts in recent days.

Mr. Renzi’s sudden assertiveness has left him open to charges of being an obstructionist and of grandstanding to score political points at home. He has criticized Brussels and Berlin — including a public feud with the president of the European Commission. He has elbowed his way into European Union policy matters such as Russian sanctions, a gas pipeline in Germany and the deal with Turkey to slow down the influx of refugees.

“I’m the leader of a great country,” Mr. Renzi said during an interview last week. “I have my ideas.”