Adamgerd,
Your argument is as follows:
1. Germany declares war on Poland;
2. The German war with Poland is a regional affair between Germany and Poland;
3. The Allies, upon activating their alliances and guarantees with Poland, declare war on Germany;
4. Therefore, the Allies are responsible for declaring a (justified) war against Germany.
What we have, in your argument, is the idea of escalation, or that the Allies were the ones responsible (justifiably so, by your own words) for turning a regional war between Germany and Poland into a world war. Important to this is the idea of the proximate cause, or the necessary condition for war. What you're missing is the critical point that the proximate cause for WW2 was German aggression against Poland, and that the subsequent Allied declarations of war are not, in any way, at fault for the escalation of the conflict for the simple reason that Germany, by virtue of its invasion of Poland, had already fulfilled the proximate cause of conflict.
Your argument breaks down immediately with point (1) and (2).
Your very first premise (1) is that Germany declares war on Poland, and only Poland. This almost immediately loses any validity as a point of contention. Imagine I go up to a police officer. I have a gun in my hand. I point the gun at the police officer. I then pull the trigger and put a bullet in the officer's stomach. He's not dead, yet (in fact, he very well may survive!), but the other police around me decide to shoot me anyways, saying I killed their comrade.
Under your argument, we'd be saying, "Well, this was initially only a little disagreement between two people that got out of hand, and it was the decision of the other police officers to turn this into something bigger". Therefore, the police officers are the ones that escalated the situation, and, even though the escalation was completely and utterly justified, the final blame for the escalation must be assigned to them.
You see, I shot the police officer knowing full well the consequences of my actions. Germany declared war on Poland knowing, with absolute certainty, that it would mean a war with the UK and France. Just like shooting the police officer will necessitate a justified reaction, Germany declaring war on Poland necessitated an Allied response.
Imagine another scenario. You're driving a car in a city. You come up to a traffic light. You know you won't make the light, so you floor it. The light turns red, you decide to just run straight through. Once across, a police officer flashes his lights to stop you, and you receive a citation. In this scenario, you knowingly violated the law by running the red light. Under your argument, you'd be saying, "Well, it was the police officer's fault for ticketing me for the sole reason that he was the one who did it. My actions merely provided the justification, whereas it was the officer's ultimate decision to make the call to ticket me".
You can see how ridiculous that is. In both examples (the police shooting and the traffic stop), you (Germany) are clearly at fault and your actions are the proximate cause for the escalation of conflict.
Let's go back to (2), or the idea that the German war with Poland is a regional affair. We know this is a fundamentally unsound premise to your argument because we know, clearly, that Germany's war with Poland was not a regional affair. Let's use another example:
Say Russia, right now, declares war on...Estonia. Estonia is a member of NATO. NATO members are obligated to defend each other from an attack. Therefore, NATO would respond to the war by declaring a state of war with Russia. Under your argument, we'd be saying, "Well, Estonia vs Russia is a regional affair, and it's NATO's fault for escalating the conflict".
It's patently ridiculous. One cannot simply ignore an entire system of alliances and political guarantees in the context of a regional war. To declare war against Poland meant that Germany would be engaged in a state of war against the Allies. The proximate cause for conflict (German aggression for Poland) is responsible for the escalation of the war, not the Allied declarations on September 3rd.