In the belief that Great Britain lacked the will to honour its treaty obligation to defend Poland, Hitler launched his powerful war machine against Poland on 1 September 1939. The Poles were taken completely by surprise when one and a half million German troops poured across their borders. Polish lancers on horseback were slaughtered as they bravely, but ineffectually, charged columns of German tanks.

The Polish army fought bravely, but was surrounded and quickly crushed by the German armies. The Polish air force was destroyed within forty-eight hours of the German onslaught. Almost all Polish aircraft were destroyed on their airstrips before they could take off, and thereafter, German bombers were able to roam freely across Poland's undefended skies and strike with impunity at military and civilian targets.

By 15 September 1939, Poland was at the mercy of the Nazis and the Polish Government fled to Romania. The Russians then invaded Poland to share the spoils of war with Germany.

As soon as it learned of the Nazi dictator's invasion of Poland, the British government acted on its undertaking to defend Poland, and demanded that Hitler halt the attack on Poland and withdraw his troops. When Hitler failed to respond, the British government declared war on Germany on 3 September 1939. This date is acknowledged as the beginning of World War II.

As a British Dominion, Australia automatically became involved in the war with Germany. Immediately following the declaration of war, Australia began to raise a Second Australian Imperial Force, or 2nd AIF, for military service overseas. Four infantry divisions, the 6th, 7th, 8th and 9th were raised between 28 September 1939 and 18 December 1940. The 6th, 7th and 9th divisions were sent to the Middle East to aid Britain in its struggle against Germany and Italy. Units of the 8th Division were sent to Malaya, Rabaul, and the chain of islands between them. These units of the 8th Division would be lost when Japan's armies began their drive towards Australia in 1942.

After the defeat of Poland, Hitler's armies then invaded and occupied Denmark, Norway, The Netherlands, Belgium and France. Hitler's ally, the Italian dictator Mussolini, declared war on Great Britain and France in June 1940 when he saw that France was close to defeat. When France surrendered to the invading Germans and Italians in June 1940, Great Britain and its overseas Dominions, including Australia, were left alone to fight Hitler and Mussolini whose armies straddled Europe from the English Channel to the border of the Soviet Union.

In August 1940, as a prelude to invasion, Hitler launched heavy air attacks on Britain. Between August and October 1940, the British and German air forces engaged in the great air war over England which is known as the Battle of Britain.

Unable to defeat the Royal Air Force, Adolf Hitler then launched a terror campaign against Britain's civilian population. Children weep in the ruins of their home after German bombers have struck London.

After initially struggling to survive the German onslaught, the British air force finally gained the upper hand and forced Hitler to abandon his invasion plan and daytime bombing. Angered by British resistance, Hitler ordered his air force (Luftwaffe) to bomb English cities at night to terrorise civilians and reduce German aircraft losses. Hitler believed that the nightly "Blitz", as it came to be called by Londoners, would cause the British to turn against their government and demand surrender to Germany. The plan backfired on Hitler. Britain now had a strong leader in Winston Churchill, and the British people rallied behind him and his government.

After one of Hitler's terror bombing raids on British cities, firefighters struggle to control fires.

Frustrated by Britain's defiance, and despite the existence of an anti-aggression clause in the Nazi-Soviet Pact, Hitler now turned his attention to his dream of conquering the Soviet Union. Stalin's occupation of the three Baltic states and portions of Romania in June 1940 had severely strained relations between the two dictators.

Japan was a natural ally for Nazi Germany and Fascist Italy in 1940 because all three countries were ruled by undemocratic, authoritarian governments, and Japan had a lengthy record of unprovoked aggression against neighbouring countries.

On 27 September 1940, Germany, Italy and Japan signed the Tripartite Pact. The agreement recognised Japan's self-assumed role in establishing a "New Order" in East-Asia, and provided for mutual assistance should any one of the three powers be attacked by another country. The Germans and Italians wanted the pact to convey a clear warning to the United States that it would face war with Japan if it entered the war in Europe on Britain's side.

On 18 December 1940, Hitler issued a fateful directive to attack the Soviet Union. It was a decision that would ultimately bring about the defeat of Nazi Germany.

While busy with preparations to attack the Soviet Union, Hitler was distracted by Mussolini's troubles in North Africa. Shortly after declaring war on Britain in June 1940, an Italian offensive was launched from Libya into Egypt. Mussolini's objective was the vital Suez Canal. In December 1940, the British counter-attacked, and routed the Italian army. By 7 February 1941, the entire Italian army of ten divisions in Libya had been annihilated. The British captured large areas of northern Libya, and found themselves with 130,000 Italian prisoners of war on their hands. Italian soldiers appeared to have no enthusiasm for Mussolini's war in the desert against tough troops from Britain, Australia, and other British Dominions. Italian soldiers needed little encouragement to surrender in very large numbers. In desperation, Mussolini begged Hitler for help, and General Erwin Rommel was sent to the aid of the stricken Italians in North Africa.

With the aim of encouraging the Japanese to attack Britain's colonies in Asia, and thereby diverting British military resources from the continuing struggle in North Africa, Hitler met Japanese Foreign Minister Matsuoka in Berlin in March 1941. Hitler urged the Japanese to attack Britain's military base at Singapore. The Nazi dictator made it clear to Matsuoka that the aim of this policy was to defeat Great Britain quickly and so keep the United States out of the war. The Japanese foreign minister agreed that the time was ripe for a Japanese attack on Singapore, but would not give Hitler a firm undertaking.

Despite his concern to keep the United States out of the war, Hitler then made an extraordinary blunder. To encourage Japan to attack Singapore, he gave the Japanese foreign minister an assurance that Germany would assist Japan if Japanese aggression produced a conflict between Japan and the United States. Matsuoka did not tell Hitler that Japan's admirals were already secretly planning an attack on the United States fleet at its Pearl Harbor base in Hawaii.