Israeli Prime Minister Netanyahu's attempt to get President Obama to join him in drawing a "red line" for the Iranian nuclear program amounts to having Obama issue an ultimatum to Iran that the US will invade if Iran crosses it. Netanyahu's idea is apparently that Iran nuclear facilities can be knocked out by an air strike, but how badly would an air strike damage Iran's nuclear program and how would Iran retaliate. If an air strike failed to do serious damage to the program, would the US and Israel send in ground troops? Iran appears to be a more formidable opponent than either Iraq or Afghanistan. If the US invaded, it would be more expensive in lives and treasure than either of those two wars.
Unlike Israel, America does not appear to be directly threatened in the short term by an Iranian nuclear bomb. It would be much more difficult for Iran to hit the US with a nuclear warhead than Israel, which is much closer. Therefore, a US attack on Iran would be almost entirely for Israel's benefit.
Israel is a close ally, but a somewhat unusual one. The closest comparison is probably to America and Great Britain in World War II, still sometimes referred to as a "special relationship." In the 1940s, America was pretty much an Anglo, English-speaking nation, although there had been waves of immigrants, mainly from other European countries -- Germany, Italy, Poland, Scandinavia -- with lesser influxes from Asia. Our President, Franklin Roosevelt, was of Anglo background and clearly wanted to help England and Churchill as they came under attack from Germany,but he felt he could not do much because of a lack of public support for going to war. Lend-lease was a start, but one that kept the US out of the conflict. The decisive event was the Japanese attack on Pearl Harbor, which brought America into the war. Arguably there was still no reason to go to war in Europe, since Germany had not attacked the US, but it suited Roosevelt, and most of the population of the US, to go to war in Europe, actually first in North Africa, while the US moved more gradually across the Pacific towards Japan. Ironically, England had gone to war with Germany because of Germany's invasion of Poland, but Poland ended up being lost to the Soviet Union.
Now we have Netanyahu trying to force the US into a war with Iran, by drawing a "red line," issuing an ultimatum to Iran that if it does X, e.g. produces 225 kg of uranium hexafluoride enriched to 20%, then the US will attack it. Should the US really do this for Israel? The US does not like Iran; Iran held the staff of the US embassy there hostage for 444 days, until the day Ronald Reagan was sworn in to replace Jimmy Carter. But if we really wanted to destroy the Iranian government, wouldn't we just invade Iran the way we invaded Iraq? If it is in our national interest, we should do it. In this case, the invasion called for by Netanyahu seems to be in Israel's interest, not necessarily ours. An Iranian nuclear attack may be an existential threat for Israel, but not for the US. It is a lot easier for Iran to reach Israel with a nuclear armed missile than the US, and the US is a lot bigger. The explosion of one rudimentary nuclear weapon in the US would be terrible, but it would not be an existential threat to the nation.
My concern is that American Jews and some American gentiles have divided loyalties. They are more concerned about Israel than about the United States. They are willing to sacrifice thousands of American lives to stop Iran from building a nuclear bomb that poses little threat to the US. Unlike Anglo Americans during World War II, Jews constitute a relatively small percentage of the US population, yet they are very influential politically and economically. They may have the political power to force the US to go to war for Israel. Many young, relatively poor, conservative redneck Americans would probably go willingly, although the majority of the US population probably would have serious reservations about going to war again in the Middle East, especially after the war in Iraq strengthened the hand of Iran. We would have fought the Iraq war to strengthen Iran by installing a Shiite regime, and then fought a new war with Iran to weaken Iran, to undo what we did in Iraq.
If Roosevelt had difficulty coming to the aid of England during World War II, despite America's close ties with her, I hope that we will have at least as much difficulty going to war with Iran when there is a much smaller group of Israel-loving Jews pressing for it.