1 May–18 Sep 1915Russian front inPoland collapses
7 May 1915German submarinesinks Lusitania withAmerican losses,angering US andleading Germanyto stop unrestrictedsubmarine warfare
23 May 1915Italy declares waron Austria-Hungary
3 Oct 1915Allies arrive in Greece
8–9 Oct 1915Central Powerscapture Belgrade
14 Oct 1915Bulgaria declareswar on Serbia