In 1939 Britain and France guarantee Poland's independence as a response to the German occupation of Czechoslovakia. Adolf Hitler's military strategists predict that Germany would not win a war with Britain and France due to the likelihood of American involvement. German diplomats meet with Soviet diplomats and rediscuss the Non-Aggression Pact, in the end agreeing that the Soviet Union would still restrict its invasion to its part of the partition even though Germany had abandoned plans to invade the part of Poland it had been partitioned. Germany still makes secret plans with the Slovak Republic, a client state of Germany to invade Poland.
On September 7th, 1939 the Soviet Union goes ahead with its plans to invade Poland. The newly created German client state the Slovak Republic invaded Poland even earlier, on September 1st.
On September 10th Britain and France declared war on the Soviet Union and the Slovak Republic. Germany sends money and arms to the Slovak Republic, but does not enter the war. The strategy is for Germany's client state to capture a significant amount of Polish territory, giving Germany more influence over territory. Because Germany and Italy will not allow French and British troops to pass through either their land or water territories Britain and France establish a treaty with Sweden to allow them to pass through to Poland.
On October 28th 1940 Italy invades Greece, prompting the intervention of Britain and France. Soon Italian troops are invading France, and France needs to order more of its forces out of Poland. The French and British military stationed in Africa invade Libya, an Italian colony. British forces in Palestine and Egypt are sent to Greece to protect it from Italy. On July 23rd 1941 Greece is fully occupied. Later that year after the United States declares war on Japan in response to Pearl Harbor Mussolini declares war on the US. During this time Italy loses Ethiopia to Ethiopian rebels with the help of the British in January 1941.
In 1942 the United States sends over troops to help in the African campaigns. In July 1942 Italy loses all territory in North Africa.
The war in Poland lasts for three years. In October 1942 the Treaty of Warsaw is signed, and the Soviet Union not only withdraws from Poland, but concedes more land to it. At the insistence of Germany the Slovak Republic does not lose any territory, although it has to pay significant reparations.
Meanwhile Italy had not only taken control of Albania and Greece but also had invaded Yugoslavia. Hungary had been partitioned between Croatia (really controlled by Italy) and Slovakia. An Italian puppet state, the Independent State of Croatia had been set up in part of Yugoslavia. Romania and Bulgaria had been allied with Italy. After the humiliating Treaty of Warsaw Italy sees no reason to keep its client states and its own army from invading the Soviet Union. Romania and Bulgaria invade the Soviet Union with the help of Italian troops. By this time Italy has changed its strategy to secure its hold in Europe, build up and possibly retake Africa later.
In February 1943 Sicily is invaded by allied forces from Africa. In May Rome is successfully invaded. Mussolini flees north, but is captured in July and executed. As a result of the invasion of Italy revolts break out in many of its holdings. Yugoslav communists take control of Croatia and Serbia and form Yugoslavia. Communists also take control of Albania. More democratic rebels seize control of Greece and are aided as allied troops come in to help. Romania and Bulgaria are captured by the Soviet Union in August, as is Hungary and Slovakia. While still a puppet state of Germany Hitler realizes that trying to aid Slovakia against the Soviets would be futile. The Soviet Union sets up a number of client states in Slovakia, Hungary, Romania, and Bulgaria.
Japan bombs Pearl Harbor on December 7th, 1941. As a result the United States goes to war against Japan. Hitler's military advisors recommend against declaring war on the United States. While Germany, Italy, and Japan have the Tripartite Pact it only requires the nations to go to war to assist their allies if the ally is attacked, not if the ally is in a war that started because they carried out an attack. However, Mussolini declares war on the US believing the US is likely to get involved in Europe eventually anyways.
Japan captures the Philippines the following year. The famous Battle of Midway ends in defeat for Japan later that year.
Without Germany as in OTL the United States has more troops to commit to the Pacific Theatre. The United States progresses towards Japan much faster after Midway. It recaptures the Philippines in March 1943, and takes Okinawa in January 1944. The Naples Conference is held and declares an ultimatum to Japan calling for unconditional surrender. However, Japan is unwilling to do this. In February the United States begins bombing Japan. Meanwhile the Soviet Union declares war on Japan invading Manchuria and liberating Korea. In March the invasion of Japan begins. This is difficult and costs thousands of lives. By May the allies decide to agree to accept a conditional surrender. Representatives of Japan meet with the allies and sign the Treaty of Moscow, which stipulates that Japan surrenders with the conditions that Emperor Hirohito will be allowed to keep his position although it is agreed that it will be a purely ceremonial position and Japan will assume a democratic form of government.