Israel blythly assumes that this will end up being Israel/USA versus Iran which, of course, would be no contest. What the USA fears is that it won't end that way i.e. Israel could end up provoking a war that pits Israel/USA versus Iran/Russia/China, which would be a monumental blood-bath.
Iran's oil exports to South Korea quadruple to 400,000 barrels a day after sanctions lift (Reuters)
from the article: U.S. army chief heads to Israel as fears over attack on Iran mount