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
3 September 1939 All Allied
Imperial possessions (includes India (modern Pakistan and Bangladesh), Ceylon, Burma, Rhodesia, Kenya, Tanganikya, Caribbean possessions, Malaya etc.
During September 1939 All Allied
Czech Government in exile ( Czechoslovakia had been absorbed by Germany in Jan 1939)
April 1940 (After being invaded by Germany)
May 1940 (After being invaded by Germany)
Oct 1940 ( After being invaded by Italy)
April 1941 ( After being invaded by Germany, Bulgaria and Hungary)
April 1941 All Axis
Croatia ( German puppet state)
Iraq ( government overthrown by British forces in 1941, thereafter Allied)
June 1941 (After being attacked by Germany)
Mongolia (USSR puppet state)
USA ( After being attacked by Japan) Allied
China ( Had already been at war with Japan since 1937) Allied
Korea (Japanese colony) Axis
all at different times
France and Great Britain
Americans felt that if they join the war they would be fighting for their country. many felt an obligation to join, or they were forced by drafts
people wanted to help their country fight for their freedom
Because Ireland was and still is a neutral country.
to end the war :)
money and lives
Mine has had enough of warfare so no.