The Second World War (WWII) was a war that lasted from 1st September 1939 to 2nd September 1945 ( though there were related conflicts which began earlier and some that went on later). The vast majority of the world's countries were involved and eventually formed two opposing military alliances: the Allies and the Axis. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort.
World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities. It included the genocide of the Holocaust, bombing that destroyed towns and cities, massacres of soldiers and civilians, starvation and disease for millions and ultimately the first use of nuclear weapons.
The Allies: In 1939 the Allies consisted of Poland, France, the United Kingdom and dependent states, for example British India, and the independent Dominions of the British Commonwealth: Australia, Canada, New Zealand and South Africa.
In 1940 they were joined by the Netherlands, Belgium, Greece and Yugoslavia (after the German invasion of North Europe).
In June 1941 the Soviet Union joined after being invaded and in December 1941 the United States joined after the Japanese attack on Pearl Harbour (though they had been providing materials before this). The Chinese had been in a prolonged war with Japan since 1937 but officially joined the allies in 1941.
In 1945, the Allied nations became the basis of the United Nations.
The Axis: The Axis consisted of Germany, Italy and Japan. The Axis members agreed on their opposition to the Allies but cooperation and coordination of their activity was not great.
Multiple sources including https://www.nam.ac.uk/explore/second-world-war