Steven Simon writes at (pdf) the Council on Foreign Relations about what an Israeli strike against Iran might mean for US interests. His list of consequences:

First, regardless of perceptions of U.S. complicity in the attack, the United States would probably become embroiled militarily in any Iranian retaliation against Israel or other countries in the region. Given uncertainties about the future of Iraq and a deepening commitment to Afghanistan, hostilities with Iran would stretch U.S. military capabilities at a particularly difficult time while potentially derailing domestic priorities.

Should Iran attempt to block the Strait of Hormuz by mining, cruise missile strikes, or small boat attacks, these fears would become realized. According to the GAO, however, the loss of Iranian oil for eighteen months would increase prices by only $6 to $11/bbl, assuming that the International Energy Agency coordinated release of reserves. This said, at the onset of the crisis, prices might hit $200/bbl (up from the current level of around $77/bbl) for a short period but would likely quickly subside.

Third, since the United States would be viewed as having assisted Israel, U.S. efforts to foster better relations with the Muslim world would almost certainly suffer. The United States has an enduring strategic interest in fostering better relations with the Muslim world, which is distinct from the ruling elites on whom the United States depends for an array of regional objectives. In part, this interest derives from the need to lubricate cooperation between the United States and these governments by lowering some of the popular resentment of Washington that can hem in local leaders and impede their support for U.S. initiatives. A narrative less infused by anti-Americanism also facilitates counterterrorism goals and, from a longer-range perspective, hedges against regime change. The perceived involvement of the United States in an Israeli attack would undercut these interlocking interests, at least for a while.

Fourth, the United States has a strong interest in domestically generated regime change in Iran. Although some argue that the popular anger aroused in Iran by a strike would be turned against a discredited clerical regime that seemed to invite foreign attack after its bloody postelection repression of nonviolent opposition, it is more likely that Iranians of all stripes would rally around the flag. If so, the opposition Green movement would be undermined, while the ascendant hard-line clerics and Revolutionary Guard supporters would face fewer constraints in consolidating their hold on power.

Fifth, an Israeli attack might guarantee an overtly nuclear weapons capable Iran in the medium term.

Sixth, although progress toward an Israeli-Palestinian final status accord remains elusive, an Israeli strike, especially one that overflew Jordan or Saudi Arabia, would delay fruitful renewed negotiation indefinitely. Both Washington and Jerusalem would be too preoccupied with managing the consequences of an attack, while regional capitals would deflect U.S. appeals to upgrade relations with Israel as an incentive to concessions. If Hamas or Hezbollah were to retaliate against Israel, either spontaneously or in response to Iranian pressure to act, any revival of the peace process would be further set back.

Finally, the United States has an abiding interest in the safety and security of Israel. Depending on the circumstances surrounding an Israeli attack, the political-military relationship between Jerusalem and Washington could fray, which could erode unity among Democrats and embolden Republicans, thereby complicating the administration’s political situation, and weaken Israel’s deterrent. Even if an Israeli move on Iran did not dislocate the bilateral relationship, it could instead produce diplomatic rifts between the United States and its European and regional allies, reminiscent of tensions over the Iraq war.