Today, it is seventy years since Germany invaded Poland, leading the allied powers in Europe to declare War on Germany (the USA remained neutral until the Pearl Harbour attack, it joined the War in 1941 and Germany responded in kind). Eleven months before Neville Chamberlain had returned to the UK proclaiming 'Peace for Our Time', having ceded the Sudetenland to Nazi Germany. The Sudetenland was to satisfy Hitler's demand for expansion - but it contained most of the defenses for Czechoslovakia. The Czechs and Slovaks call the agreement 'The Munich Dictate'.
France and the United Kingdom told Czechoslovakia that they would be alone if they did not acquiesce to the agreement.
In March, Germany invaded Czechoslovakia, and on March 21st, Hitler demanded that 'The Free City of Danzig' be returned to Germany. Following World War 1, Danzig became independent of Germany, and totally surrounded by Poland. Today, we know Danzig as Gdańsk, and it forms part of Poland.
On March 31st, France and the UK 'guaranteed' Polish independence.
In April, Hitler ordered the Germany military to plan for Fall Weiß, the attack on Poland. Shortly thereafter, Italy invaded Albania and the Soviet Union formed an alliance with the United Kingdom and France.
At the end of April, Hitler renounced the Anglo-German naval agreement and, ominously, the German-Polish non-aggression pact.
In May, Sweden, Finland and Norway reject Germany's offer of non-agression pacts.
The Pact of Steel was signed between Italy and Germany, ensuring each would assist the other in the event of War. Mussolini informed Germany that if the planned attack on Poland went ahead as scheduled, that Italy would not be ready. This delayed the Polish attack by a week, as Germany was trying to get neutrality agreements with France and the UK. Italy did not join the war until June 1940, when it attacked Southern France. The Italians signed an armistice with the Allies in September of 1943, though Northern Italy, under Mussolini, continued to be a member of the Pact in name only.
In August of 1939, the Molotov–Ribbentrop Pact was signed between Germany and the Soviet Union, it was the 'Treaty of Non-aggression between Germany and the Union of Soviet Socialist Republics'. It ensured that Germany and the Soviet Union would remain neutral if the other were in a conflict. It lasted until June 1941, when Germany invaded the Soviet Union in Operation Barbarossa. The Soviets sought the pact as Hitler had talked about expanding to the east in Mein Kampf, and they sought to prevent that. The agreement also divided Eastern Europe into Soviet and German 'spheres of influence'. Of the countries mentioned, only Finland managed to remain independent, repelling the Soviet attack of November, 1939.
On September the 1st, Poland was invaded, without a formal declaration of War. An ultimatum was issued by Chamberlain, which expired at 11am on the 3rd September. He announced that 'consequently this nation is at war with Germany'. France issued at ultimatum a few hours later. Interestingly, the USA has only declared war formally five times (the last time was World War 2). I've been unable to find an equivalent article for the United Kingdom.
The first action of World War 2 which concerned the UK directly was the German torpedoing of the SS Athenia on the 3rd. The ship was en route from Scotland to Montreal. The first British action was the targetting of the Admiral Scheer, a battleship which was in Heligoland Bight. This was the 4th September. The ship survived.
Japan and the USA declared neutrality in Europe on the 4th and 5th respectively. South Africa, following a failed attempt to establish neutrality (involving a change of Prime Minister) declared War on Germany. This was on the 6th. Canada declared war on Germany on the 10th.
The first British military personnel killed was at the Battle of Barking Creek on the 6th September - this was a false alarm and a 'friendly fire' incident.