Germany and the Soviet Union invaded Poland together, Britain only declares war on Germany which began WW2...
Nope. Germany invaded Poland first, they didn't invade together, and Hitler invaded Poland with full knowledge of the consequences; that Britain & France would be brought into the war. The pact between Britain and Poland was signed before the invasion, and the German invasion was even postponed because of it.
The fact is, one of the primary causes of WWII was the Treaty of Versailles, and one of the primary 'sticking points' for the German nation was that they lost the Danzig corridor. The start of WWII is thus offically marked by the German invasion of Poland, which triggered the involvement of Poland's allies.
The mistake you're making is to equate a declaration of war with the start of a war itself. By that logic, the US was never involved in the Vietnam War, because they never declared war on North Vietnam or the Viet Cong.
Likewise, war between Japan and the US started with the attack on Pearl Harbor, not when the official declaration of war was made by either party.





















































