With what? And how do they propose to get there?
Funny, even in the Middle Ages the Crusaders managed to "get" to the Middle East. I know that high tech wars require logistics, but Europe has plenty of planes and ships, both military and civilian. Where there is a will, there is a way. I think Europe fighting is more a question of will than anything else.
Not to meantion the US controlls two nations next to Iran, getting there will not be a problem.
I honestly think that invading Iran and toppling its government is well within the capability of existing U.S. forces, but that occupation of that country for any appreciable amount of time is well beyond that capacity. Occupation of Iraq took approximately 220,000 troops.
But an Iranian campaign may not look like Iraq. It may well more closely resemble Afghanistan, in which Special Forces are emphasized and the occupation is of key facilities only. Iran is certainly gearing up for that sort of confrontation. Its sober commanders realize what would happen should they decide to roll the tanks toward Baghdad, the commanders that Ahmadinejad hasn't had killed yet, anyway. Iran's emphasis will be on defense of the homeland until they can get a nuclear bomb and probably long thereafter as Europe's will to confront them fades and a confrontation with Israel is delayed in favor of heavy support for Iran's proxy armies in Lebanon. That's my best guess at their strategic plan.
In addition, pressure will be brought to bear on China and Russia to oppose the U.S. for the sake of opposition and (in China's case) a guaranteed oil supply. Even now it is highly doubtful if either of those countries feel that an Iran without the mullahs is preferable to one with them and its resultant opposition to the U.S. That is certainly how they have behaved with respect to North Korea and I don't see it changing. I would truly not look for any help from that area. Russia feels it has the land, and China the population, to withstand any serious military threat that the Iranians can muster now and forever.
If I seem pessimistic it's because I am. This could have been a showcase for international collective security had not those institutions fallen into corruption and irrelevance. As it is it represents the best opportunity for China, Russia, and Europe to return to a Great Powers geopolitical arrangement with the U.S. spending its own power in pursuit of a chimerical settlement of the issue that is in neither China's or Russia's interests and that the Europeans have convinced themselves incorrectly isn't in theirs either. Nations don't have friends, only common interests, and I'm afraid it looks as if we just don't have enough common interests with the ones we need to help us resolve this with a minimum of pain.