When did each country join World War 2?

One has to define the beginning of WW2. Several regional conflicts existed through the 1930s. Italy's campaign in Ethiopia and Japan's undeclared war against China for instance. The usually accepted date that WW2 commenced is 1 September 1939 when Germany attacked Poland. Britain and France declared war on Germany on 3 September and at the same so did the constituent members of the British Commonwealth. Other European counties, later members of the Allies, entered the war only when attacked by the Axis. The US only entered the war after the Pearl Harbour attack at the end of 1941, as did US client states such as Mexico and Cuba. As the war swung to the advantage of the Allies, even fence sitting countries or those friendly to Germany and Japan eventually joined the allied cause--Argentina for example declared war on Germany in March 1945 and Chile in April 1945 three weeks before the war had ended.

As the war progressed some countries changed sides. Finland, Hungary and Bulgaria were initially Axis but switched 1n 1944 when German a defeat was inevitable.

Many people whose countries were nominally neutral joined the forces of various countries. Hundreds of thousands of Irish fought mainly in the British forces. After the fall of France, many Frenchmen joined the Free French forces and some joined the Nazi SS as did citizens of Latvia, Lithuania, Estonia and some Balkan countries. Many Palestinian Jews joined the British forces and others mounted terror campaigns against British troops.

Small numbers of US volunteers fought in Allied units before the US entry into WW2, most notable were the Eagle squadrons of the RAF and the Flying Tigers in China.

The full list of countries is too extensive to list here, but the main countries, their date of entry and which side are here.

1 September 1939

Germany (including Austria) Axis

Slovakia (German puppet regime) Axis

Poland Allied

3 September 1939 All Allied

United Kingdom

Imperial possessions (includes India (modern Pakistan and Bangladesh), Ceylon, Burma, Rhodesia, Kenya, Tanganikya, Caribbean possessions, Malaya etc.

Australia

New Zealand

France

During September 1939 All Allied

Canada

South Africa

Newfoundland

Nepal

Czech Government in exile ( Czechoslovakia had been absorbed by Germany in Jan 1939)

April 1940 (After being invaded by Germany)

Norway

Denmark

May 1940 (After being invaded by Germany)

The Netherlands

Belgium

Luxembourg

June 1940

Italy Axis

Oct 1940 ( After being invaded by Italy)

Greece Allied

April 1941 ( After being invaded by Germany, Bulgaria and Hungary)

Yugoslavia Allied

April 1941 All Axis

Bulgaria

Hungary

Romania

Croatia ( German puppet state)

Iraq ( government overthrown by British forces in 1941, thereafter Allied)

Finland

June 1941 (After being attacked by Germany)

USSR Allied

Mongolia (USSR puppet state)

December 1941

USA ( After being attacked by Japan) Allied

China ( Had already been at war with Japan since 1937) Allied

Japan Axis

Korea (Japanese colony) Axis

1942

Brazil Allied

Thailand Axis