You're overdoing this.
Iran attacking Israel would be sorta like the Lebanon war just a bit more serious.
A lot more serious. Israel is supported by the US. Iran is supported by Russia. The European Union has an uneasy relationship with Russia at the moment, with Putin and Medvedev tweaking them over natural gas prices and pipelines.
Russia, for its part, is experiencing significant economic disruption. A war would go a long way towards boosting their economy and pulling idle workers off the streets.
China would enjoy the export markets military adventurism would open up.
The effect of globalization of the world's economies takes on a certain levels disturbing parallels to the interlocking military treaties that existed in Europe before WWI. Given the right impetus, a great many nations could be pulled into war whether they wished it or no.
It wouldn't lead anyone to actually join into the war, no one has the interest to do so.
If the US did not defend Israel, it would have no credibility as a diplomatic force in the Middle East. Additionally, failure to defend Israel would almost certainly doom whomever might be President to electoral defeat in the next election cycle. The US does not desire war in the Mid East--but it may not have the luxury of avoiding it should Iran strike at Israel.
If the US defends Israel, if Russia did not aid Iran then Russia's political clout in that part of the globe is diminished; Putin's hegemonic and imperial ambitions are abundantly clear--if he wants to continue that path he must support Iran if Iran finds itself in conflict with the United States.
The Arab nations would likely oppose an Israeli strike on Iran. Getting them to sit on the sidelines in such a conflict strikes me as nearly impossible. If the US is defending Israel against Iran, and the Arab nations weigh in on the side of Iran, NATO countries are going to find it next to impossible to stay on the sidelines (they need ME oil as much if not more than we do), as will China.
It is in nobody's interest to have a war in the Middle East. It was in nobody's interest to have a war in August, 1914, either.
Perhaps Hizbollah and Hamas would launch some rockets, but I can't see those rockets doing any serious damage.
As long as Iran works through the Hezbollah and Hamas groups as proxies, this is a plausible scenario. A direct attack by Iran on Israel is a completely different situation.