Nothing short of all-out war will stop Iran
If we continue sanctions, without a military strike, Iran will eventually make nuclear weapons and use them against the U.S.
If Israel destroys Natanz uranium enrichment plant, but not the underground facility at Fordow, Iran will use Fordow to purify the 100 plus kg of U-235 that they already possess, to make a few nuclear weapons. Then they will use the attack on Natanz as the justification to use those nukes against the U.S.
If the U.S. destroys both Natanz and Fordow (near Qom), Iran can use the thousands of kilograms of uranium ore concentrate and low enriched uranium that they posses to make many dirty bombs. The attack on Fordow will be portrayed as an attack on the holy city of Qom, and therefore as an attack on Islam. Iran will then use the idea of a holy war to attack the U.S.
Iran might have one or more other nuclear facilities, unknown to the West, to complete the enrichment of uranium and the production of nuclear weapons. They have publicly declared that they intend to build more enrichment facilities.
If the West attacks Iran, the Iranian regime might garner some support from North Korea or Pakistan, so as to obtain highly enriched uranium (HEU) from one of those nations. They might be able to obtain HEU from China, which has long supported Iran’s nuclear program.
No matter how each scenario plays out, the end result is a major war between the U.S. with our allies and Iran with its allies. But if we attack Iran first, the war will do much less harm.
A first strike to begin a major war is morally justified, if (1) the war is undertaken with only good intentions, (2) and is ordered toward the defense of the innocent, (3) and with the reasonable anticipation that the war will not do more harm than good. In this case, if we attack Iran with a sudden and severe attack against nuclear facilities, the Iranian military, command and control targets, and other military targets, we may be able to completely eliminate their ability to attack the West with nuclear bombs or dirty bombs. The war would be much shorter, cost many fewer lives on both sides, and nuclear devastation would be prevented.
All other options end with Iran using nuclear bombs or dirty bombs against mass centers of the civilian population in the U.S. and other nations. It is not moral to use sanctions alone, since it is now clear that this approach will not work. It is not moral to use only a very limited military strike, due to the great likelihood that Iran will counter attack with nuclear weapons or dirty bombs.
A first strike is the only moral option.