Possibly China!!! They were invaded by the Japanese in 1937, two years before WW(II) started .
The Japanese and Koreans were exceptionally cruel to the Chinese, by way of torture, starvation, dehydration , rapine of women , imprisonment. This did not come to an end until August 1945, when the USA dropped the 'Bombs' on Hiroshima and Nagasaki.
Britain, France, Poland, Russia and many others. Officially, WW2 started on 1 September 1939 when German forces invaded Poland. Unofficially, WW2 started sometime in 1931 when Japanese forces invaded Manchuria after the Mukden incident which was staged as a prelude to invading China in 1937. American pilots and soldiers were in the thick of it by 1937. Some famous individuals were General Chenault of the Flying Tigers. On the ground it was a brave band of soldiers 'Merrill's Marauders" who fought from 1937 to 1945. Some of these soldiers went on to fight in the Korean conflict. Pappy Boyington started out with Chennault in Burma before taking over a rag tag group at Espirutu Santos in the Pacific.
So your question is really about who you want to look at. The Americans officially went to war on December 7th 1941. The Russians officially in June 1941. The Chinese would say 1931. The British on 3rd September 1939. The Ethiopians in 1937. The Finns fought against the Russians from 1939. Many Allied countries although their country had surrendered, continued fighting under their flag like the Free French Forces under General de Gaulle.
Not every country fought in World War II. The main countries that fought in World War II were the US, UK, France, and China, versus Germany, Italy and Japan.
World War I was fought between 1914 and 1918.
World War 1 was fought against primarily Germany.
Most of World War 2 (but of course not all) was fought in Europe.
Most of the world fought in it.
Vietnam
It was Great Britain.
Depending on who you are you fought in World War 2. World War
we fought japan,germany,and russia we had a cold war.
Not every country fought in World War II. The main countries that fought in World War II were the US, UK, France, and China, versus Germany, Italy and Japan.
No country is still at war with Germany...
Italy
The United States was the country that entered World War 1 late. The war was fought from 1914 to 1918.
USA & Borneo,
The Hundred Years War, with the English against the French from 1337 to 1453, thus lasting for 116 years.
The Battle of Verdun, 21st February to 18th December 1916, was the largest and longest battle of the First World War. It was fought between the German and French armies.
Spain was a Neutral country in World War 2, and did not fight on either side.