I'm not that fond of the CIS, but I don't like how everyone assumes the Jedi side is always right. So from what I can tell, the Trade Federation and Count Dooku have been causing most of the trouble in terms of the assassination attempts. I don't think three spies, despite their importance, should not be the cause of a galaxy wide war which will lead to the death of billions of lives. If the CIS attacked first, then of course they should be able to fight back but they had little confirmation of any invasion taking place and used no diplomatic weight to tell them to stop making such large armies, they just charged in.
Again, you are ignoring the fact that Obi-Wan heard the plans of the C.I.S. leadership., which was to attack the Republic and overwhelm the Jedi, forcing the Senate to agree to any demands they make. Why is this not getting through?
Those 3 weren't the reason for the war. War was inevitable, because it's what Dooku and Sidious wanted. Obi-Wan reported this to the Council, the C.I.S. had no intentions of doing anything but attacking the Republic. This is why they gathered the Jedi to save Obi-Wan, Anakin, and Padme : War was inevitable, the C.I.S. was building a huge army for that purpose alone, the Jedi would save those 3 and buy time for the Clone Army to show up.
If you know an enemy is preparing for a massive attack, and you have the chance to strike first (while saving 3 incredibly important figures in galactic affairs), you're going to take it.
The droid army wasn't really illegal if the CIS was actually a Confederation of Independent Systems; they should be able to have their own military like the Republic had on a smaller scale to deal with local crises and pirates. .
The Trade Federation provided most of the droids we see in Episode 2 (minus the Super Battle Droids). The Federation was banned from producing a droid army after Naboo. It was illegal.
What I am basically trying to say is that the Republic should have tried more peaceful tactics before invading Geonosis and launching such a destructive war.
Heh, you missed the entire backstory for Episode 2 I guess. The Republic tried negotiations for years, as soon as the C.I.S. was formed and announced their intentions to leave the Republic. But guess who the leader of the Republic was? A Sith Lord! Kind of, you know, manipulating events to push the galaxy into war.