Iran test-fires missiles, sinks target not far from US-led international fleet.

In no way are they defenseless. If the US was going to strike Iran, they would definitely be the ones to initiate. Israel would be part of the secondary offensive, but not the primary. War with Iran would be a large scale offensive, and we would not want those in the Israeli military or NATO to blow the whistle and ruin the element of surprise. Therefore, the US would start by moving strategic forces into position slowly in the various land bases in the region. They would most likely have two carrier groups moved to the waters off Iran, albeit out of missile defense range. Guided missile cruisers and ballistic missile submarines would then move into striking distance of Iran's coastal defenses in preparation of an initial strike. They would target all known radar and ballistic missile sites. The US has 22 active guided missile cruisers. They also have 4 active guided missile submarines, apart from the other 14 nuclear warhead carrying subs, that each hold 154 tomahawk cruise missiles. Most likely they would keep the surface missile cruisers out of range during the initial strike, and mostly rely on land based ballistic missiles, and the 616 missiles on the four Ohio class submarines. They would probably only move the surface missile cruisers closer once Iran's defenses have been bombarded by air and land, in protection of the two carrier groups, who will then launch their aircraft as well. Then it really depends on our directive what happens after that. Is there a ground based invasion of Iran, or will we stop once we have destroyed their nuclear facilities and missile capabilities? If we proceed with a ground-based invasion, then we are talking a serious endeavor which would be very costly. I do not see a full ground based invasion as a possibility.

EDIT: I really don't see this happening given the current situation. My biggest fear would be a unilateral strike by Israel that would force our hand in the matter.