Most Powerful Countries of World War II

Germany

The Germans made the first operational jet, the largest European battleship ever, the first ballistic missile, the first radio-controlled bomb, the Gustav and Dora guns, etc. The Germans were only defeated because of their terrible allies, the fact that they were basically against the world, and because Hitler made poor military and social mistakes, including the Holocaust, which wasted manpower and money.
When it comes down to it, Germany had better technology and training. The allies only had sheer numbers. The Germans took out the 5th largest military in 20 days (Poland) and killed well over triple their casualties. The Tiger tank took out 5 Sherman tanks for every Tiger. They probably had the top 3 tanks in the war. That is why I voted Germany for the top WWII country.

Soviet Union

The Soviet Union and China did all of the heavy lifting during WWII, and I'm sorry, but that's just true. Seventy percent of all Japanese casualties were caused by the Chinese, and seven out of every eleven German soldiers that died were killed by the Soviets. And Germany was not going to win even if they captured Moscow. Stalin would not let them take Moscow.
The production capabilities and the sheer manpower of the USSR were enough to beat the Germans. And the reason they were not going to surrender if Moscow was taken is that Stalin was going to put every single person in the USSR between him and the Germans, and the Germans were not going to win WWII even from the start. Hitler was experiencing manpower shortages as early as 1942 or 1943, this mainly being caused by the eastern front.
So I guess I'm just going to put my list here.
1. Soviet Union
2. United States
3. United Kingdom
4. Nazi Empire
5. Japan
Honorable Mentions: Canada, France, China, Italy, and the rest of the dominions of the British Empire

United States of America

Although the German Reich invented the first operational jet and defeated most countries it was up against, their problem was oil. The U.S. was the number one producer of oil. Since the Germans decided to fight a war of attrition, they were doomed. Germany could not win World War II.
As for the Soviets in the east, in 1941, the Soviets were disorganized and had terrible leadership after the Great Purge in the 30s. They particularly favored strength in quantity over quality. If the U.S. were to execute 'Operation Unthinkable,' the Soviets would certainly push the Allied powers to the Rhine, and possibly to the English Channel. The United States had the resources, the manpower, the industry, quality in equipment, and the allies. The U.S. would ultimately dominate the entire scene in World War II.

United Kingdom

The US did not win the war for us. Despite popular propaganda, most historians that aren't tainted by patriotism will tell you the US profited off war bonds. While their impact is far from small, it holds nothing against the 4 years of young British blood spilled fighting the Germans. We were by far the prominent force, with Russia wavering and France getting its ass kicked.
The war was set to be won. It was clear the population of Germany was beginning to revolt against Adolf Hitler. The US just allowed veterans of the UK army to be moved to the front line, allowing the war to be quickly finished in under half a year after that.

Japan

Pretty decent. Probably the second most efficient navy in the world, despite being smaller than Britain's. It dominated the Pacific early on.

Japan was one of the superpowers during the war. It had a very good fighting force even though lacking in numbers. They had very good training and discipline. They went through China very fast with a lot fewer troops. Their air force was okay but not that good. But what makes them so powerful is their navy. Their navy helped them take over much of Southeast Asia and was incredibly powerful with very well-made ships such as the Yamato.

Canada

Canada had the fourth-largest navy and third-largest air force. It was the most mechanized force in the world. They protected the Atlantic without US help for a big part of the war. Their fighters protected Great Britain, and their troops invaded one of the five beaches of Normandy, invaded Italy, and liberated Belgium and the Netherlands.

Canada was a little weak and strong at the same time. Its land is so big, but just sending troops to the Allies helped a lot.

Canada doesn't get invaded, but instead sends troops to Britain at the battlefield.

Italy

Oh, mamma mia! Italy planned to invade its neighbors, including sea-border countries like Albania. Albania was the first step toward reclaiming old Roman territory, and Yugoslavia completed that plan.
At first, things went well until Greece used the power of Zeus and struck at Italy, just giving them a taste of defeat. But Italy eventually lost due to the British and Americans, who caused revolts and civil wars.

Great bully in African countries, weak in defending themselves or invading others.

China

During the battle with Japan, China had pinned 90% of Japanese infantry, 50% of their air force, and 20% of the Japanese navy, with most of them being captured or eliminated! Despite a colossal number of casualties - 20 million, composed of mostly civilians and soldiers - the most populated country has never given up on fighting against the Japanese. With a large disparity between the production levels and quality of the weapons, on average, the death of three Chinese soldiers could exchange one death of a Japanese soldier. Massive respect!

It was still the real China, the Republic of China, by that time. So, the flag is wrong. The ROC National Revolutionary Army at that time used German military equipment. Its ranking should be much higher, at least higher than Japan.

Poland

Poland's military was decent before the war. When Germany's invasion started, the Polish military was able to mobilize thousands of troops quickly. Poles fought hard, but they did not stand a chance against the superior German technology and then an invasion by the Soviets.

Notable for their refusal to roll over and die, they always fought the occupation, not the most powerful, but one of the most determined.

Poland held off invaders for hundreds of years until the Germans took them out in 20 days.

India

Actually, India should be at number 4 because 80% of British troops were actually Indians. Indians also had unique tactics and tremendous strength. Had it not been for India, Britain might have lost the South Asian front.

India was a very powerful country even before and after the invasion of Britain. All of the funding for WW2 was coming from only India. Indians were a huge part of the British Army.

India had a large population and defended itself from Japan.

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Ottoman Empire

These weren't in WWII. If you meant WWI, they got destroyed and if you meant Turkey, they declared war on the last second and contributed nothing

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Nepal

Send support through tibet to china and punched well above their weight.

France

France was supposed to be number 5 or 6, but because its intelligence wasn't enough to be strong against the Germans, it fell.
Then it became unstoppable. A civil war also started when Free France revolted against Vichy France. Vichy France lost, including during a German and Italian invasion of France.

Oh! France was a bit weak and dumb but also strong. Why dumb? Because they didn't build a protective wall at Belgium and instead built it at the German-French border.
France lost its mainland and battles, but not the war. They never gave up and eventually took revenge.

Greece

They fought to defend their country from invaders (Axis). They might have lost when Germany helped Italy to take Greece, but they fought like heroes.

Greece... hmmm... ah yes. Greece held out. They demolished the Italians until Germany came along, but still, they were good. As a person above me or below me said, "Greeks don't fight like heroes, heroes fight like Greeks."

Greece fought off Italy until Germany happened.

Australia

They are some of the only ones able to stop the Japanese from invading their mainland.

Ethiopia

This country was the only African country that successfully resisted colonial domination and maintained its independence. The Battle of Adwa speaks it all.

Russia

Should be higher. Germany invaded Russia, but then Russia got its land back!

Brazil

Brazil is powerful in only one way: its population. This country could have built a powerful, extremely huge army in days.

Belgium

It's one of my favorite countries.

Spain

Franco is considered Spain's best leader. He had almost all of Spain's support. He also had German and Italian support, but even without these powers, he still had the power and support to unite Spain.

Powerful enough to mind their own country (civil war) during the bloodiest war happening around them.

Philippines

I love my country. Nationalism is in my heart and mind.

The Philippines is actually considered strong, as their nationalism is very strong.

Philippine patriots were strong in WWII. They cared about their country and they died for their country.

Vatican City

Vatican City, officially Vatican City State or the State of Vatican City, is a walled enclave within the city of Rome. They were never annexed by Italy.

Turkey

South Africa

Fiji

Romania

Romania shortened the war by 6 months by switching sides. That prevented Hitler from finishing the nuclear bomb.

Yugoslavia

Yugoslavia's military was good, but political instability was their downfall.

It was one of the strongest countries in Europe.