text: The PSRI supported Italy's entry into World War I and gave vital support to the nationalist government led by Antonio Salandra. When Italy declared war on Austria-Hungary in May 1915, Bonomi volunteered in the Army and was sent to the front as a sub-lieutenant in the 7th Alpini Regiment, taking part in the fighting. In June 1916 he was appointed Minister of Public Works in the Boselli cabinet and held this position for twelve months, until his resignation due to disagreements on domestic policy. He would hold the same Ministry a second time in the Orlando cabinet from January to June 1919. In the war's aftermath he was Minister of War (March 1920 - April 1921) and then briefly Minister of the Treasury under Prime Ministers Francesco Saverio Nitti and Giolitti.
  Crisis of the Italian democracy and first term as prime minister
  A few months later, he became Prime Minister of Italy for the first time, in a coalition government, and was the first socialist to hold the post. Early in 1922, his government collapsed, and he was replaced as prime minister by Luigi Facta, amidst the Fascist insurgency led by Benito Mussolini.
  During the Fascist period
  With the consolidation of the Fascist regime, Bonomi withdrew from political life. In the following years he devoted himself to historical research, publishing a number of articles on Giuseppe Mazzini and the 1849 Roman Republic, and working on a history of Italian politics from 1870 to 1918, which would only be published in 1944.
  Towards the end of 1942, as Italy was facing military setbacks on many fronts in World War II and growing political dissent at home, he reprised clandestine contacts with anti-fascists from various political parties. He also began the publication of the underground newspaper la Ricostruzione.