Israel will be able to gain the support of the USA, what form this support takes would depend on the situation but certainly at the least they could count on material and financial support from the US,
also perhaps Israel may find support from Turkey as both nations have grown close over the last 20 years and certain treaties and agreements are in place between them.
It is unlikely that the EU as a whole will do much, perhaps individual nations may offer support on an ad-hoc basis but Israel would not expect to get much from that direction.
It is doubtful either China or Russia would offer much more than encouragement to Iran, neither nation is in a position to offer much, Russia is simply too weak militarily to run the risk of an extended out and out shooting war against a large 1st rate military so far outside of their own borders (especially if their involvement dragged the US into direct military conflict).
China would make noise in the UN but again simply has too much to lose by getting involved and much more to gain by sitting the war out then coming in when it is finished to pick up the pieces. (China makes its progress by always taking the middle path, never picking a side on anything unless it has no choice and is sure it will gain in the long term, and as such manages to keep on reasonably good terms with the big world players, taking a side in such a conflict will either alienate the west or lose them the middle east, better to keep out)
Iran is unpopular among the other arab nations, who like the West have no desire to see Iran become a nuclear power and as such the most powerful player in the region, so it is doubtful any arab nation would actively side with Iran in any meaningful way (preferring instead to see the threat from Iran to themselves dealt with by others).
what Iran could count on however would be the support of many millions of individual muslims around the globe, these would range from Palestinians stepping up their direct war against Israel, to attempts or repeats of the 9/11 or 7/7 attacks on the supporters of israel, i.e the west.