Israel invades the gaza strip.

Sounds more like Mid-East War to me.

Israel and the United States are major allies, both with nuclear capabilities. Israel is the only democratic country in the Middle East, which gives the U.S. incentive to back and support them. Iran is working towards having nuclear capabilities (I'm not going to say weapons because of the dispute over the issue) and is a country of concern in the Middle East.

WMDs are a big deal, enough that President Obama told the Syrian government that the use of WMDs would be crossing the 'redline' and would prompt military intervention in their civil war. If Hamas and Iran go overboard and use WMDs on Israel, this will be a major issue for both the United States and the U.N. Security Council, who already both have sanctions on Iran. Russia trades with Japan and for a lot of reasons doesn't want to see the end of the Iranian regime. China is Iran's largest importer of oil (the EU is the second), so the Chinese have a stake in pretty much anything that involves Iran.

So there you have it. The EU, US, China, and Russia all have stake in Iran, not to mention the Middle East which is a huge provider of oil. If a large scale war breaks out there, you could potentially see the situation quickly escalate into a larger problem.