Axis
The Axis was the coordinated military forces of the Germany, Italy, and the Japan during World War 2.
History
The Axis alliance was initially negotiated in 1936 around the Anti-Comintern Pact, of which the original signatories were Germany and Japan. Italy joined in 1937 on condition that it was recognized as an original signatory.
Hungary and Spain joined in 1939.
The Molotov-Ribbentrop Pact significantly shifted the balance of power. As a geopolitical rival of the USSR, Japan had joined the Axis in part to seize Russian territories. The unilateral directive to maintain peace soured relations between Japan and Germany. (Operation Barbarossa of course invalidated this pact.)
The Tripartite Pact became the new foundational treaty in 1940. Germany, Italy, and Japan were the original signatories.
Hungary and Romania quickly joined. Bulgaria joined in 1941.
Yugoslavia also joined in 1941, though this triggered an Allies-aligned coup, so Yugoslavia was in fact invaded by the Axis and partitioned into client states.