The Tripartite Pact, an alliance commonly known as the "Axis", consisted of Germany, Italy and Japan. Hungary, Romania, Bulgaria and Yugoslavia eventually signed the pact as well.
Romania didn't leave the Warsaw pact, the pact was dissolved on 1 July 1991.
The Warsaw Pact.
The three major Axis powers, Nazi Germany, Fascist Italy, and Imperial Japan were part of a military alliance on the signing of the Tripartite Pact in September 1940, which officially founded the Axis powers.
The Warsaw Pact.
Germany and Italy made the tripartite pact (Axis pact) before the war. Soviet Union was also in a pact with Germany but when these invaded the SU they switch sides (to allies). Hungary, Romania and Bulgaria had Nazifriendly governments and were allied in the Axis. The occupation of European countries resulted in some new Axis alliances, like Vichy-France. Thailand entered in alliance with Japan. Japan had some puppet states in China, Manchuria, Mongolia, Philippines, Vietnam, Cambodia, Laos and Burma,
The alliance was commonly known as the Axis Powers, based on an earlier agreement, the Pact of Steel between Germany and Italy, which established the "Rome-Berlin axis" (known after 1940 as the "Rome-Berlin-Tokyo axis").The Axis was formed on September 27, 1940 under the Tripartite Pact of 1940 (not to be confused with the Tripartite Agreement of 1936, a financial agreement between the US, France , and the UK).
The pact of steel was the formalised rome berlin axis from 1936
Axis Pact
Because they were invaded by Germany (and other axis powers). They had formerly had a "non-aggression pact" with Germany, which Germany broke.
The alliance they formed was called the Axis Powers. Britain (aided by the French resistance), the U.S., and Soviet Union, were known as the Allies.
Usually the Axis powers are referred to as members of the Tripartite Pact, hereby implied that there are three main powers: Germany, Italy and The Empire of Japan. Other, later members include Bulgaria, Romania and Finland, with several other countries, such as Thailand, playing smaller roles in the Axis alignment.