Well, it happened and if I had to play my example further: USA attacks Iran. Iran is helped by Russia and Russia declares war on the US. Therefore EU declares war on Russia. WW3! So it happened in 1914.
But again the question behind was difficult policy. Austria annexed Bosnia. That was against some nationalists in Serbia, who also radicalised. They became terrorists, as Serbia was not strong enough to fight Austria in a war. So the Black Hand organized to kill the Archduke. Who was quite popular, as he was very liberal and thought to someone able to stop the decline of Austria. However killing him should destabilize Austria. If the Russian secret service played also a role in the drama is not sure, as Russia backed up Serbia. Nicholas II. had severe problems because he did not grant an at least constitutional monarchy. He was not willing doing so. Also after the loss of the war with Japan he feared losing the status of a major power if he did not help the Serbs, as he wanted to become the ruler of all Salvs. Germany backedup her last trustworthy ally, Austria, but only to a certain extent. They gave a card blanche but wanted negotiations. That was misused by Austria in the end.
Austria however should have declared war at once and nothing happened (except to Serbia). But here also interior policy played a major role. The whole state was disintegrating. Another minority would have fastened up that. So Austria (in this text I ever mean Austria Hungary), which felt unfair treated in the last big negotiations, made an ultimatum to Serbia. And yes, indeed it was accepted- except the point, that Austrian and neutral policemen should investigate in Serbia. And although this was not as common as today, that was nothing new, as the Serbs once investigated in Austria, too. As big parts of the government were involved, that could not be accepted. So this was denied and Austria declared war. Russia declared war on Austria and Germany had to declare war on Russia.
France was eager for revenge and there a normal dealing with negotaitions were not possible as France wanted the war, too. Britain and Germany had both no interests in a war but were caught in their alliances.
That is a short summary. I also posted this before, so you can read there.
However my example from yesterday tro explain it better:
Russia is backing up Iran to get more power in the deals with resources (Oil, gas). Iran is developing nukes and has some other problems with the west. Then Ahmedinedschad send a killer and he kills Condi. Bush demands an investigation and although most points of the ultimatum are accepted, no US FBI guy is allowed to investigate in Iran. So the US attack Iran. To lose no ally and trustworthiness Russia declares war on the US, who now ask the NATO for help, so that NATO and EU declare war on Russia and Iran.
After that mechanism ww1 started.
Adler