The Greek–Serbian Alliance of 1913 was signed at Thessaloniki on 1 June 1913, in the aftermath of the First Balkan War, when both countries wanted to preserve their gains in Macedonia from Bulgarian expansionism.

Preliminary discussions quickly bore fruit, and on 5 May Koromilas and the Serbian ambassador to Athens signed the first protocol, where both sides pledged mutual support against Bulgaria should the latter refuse to acknowledge the territorial status quo.

The Serbian government however refused to ratify the latter, insisting that Greece take on itself the previous Bulgarian pledge to provide 200,000 troops in the case of an Austrian attack.

The attendant military convention obliged each country to come to the other's aid with all available forces should either be attacked by a third power, or, if one of the signatories declared war first, to maintain a favourable neutrality and conduct a partial mobilization (40,000 men for Greece and 50,000 for Serbia).

The crisis between Prime Minister and King over the issue of the country's participation in the war led to the National Schism, which plagued Greek political life until the 1930s.