The president's "red line" is the point at which Iran "possesses" a nuclear weapon, which would not arrive for a couple of years even if Iran actually intends to make one. (American and Israeli intelligence services concur that it is not working on one now.)
Mr Netanyahu's "red line" comes much sooner: whenever Iran has enough enriched uranium to build a bomb, whether it does so or not.
It is, of course, quite legal for Iran to enrich uranium (which it says is solely for use in civilian nuclear reactors), while an unprovoked attack on Iran would be a criminal act under international law.
But that didn't stop former president George W Bush from invading Iraq, and it wouldn't stop Mr Obama now.
What worries Mr Obama are three other things. First, the American public simply isn't up for a third "war of choice" in 10 years in the Middle East. Secondly, this is presidential election year in the US. If Israel attacks Iran, the oil price will soar and kill the economic recovery Mr Obama is depending on for re-election.
However, if the US fails to back Israel, American Jews will turn against him and kill his re-election chances anyway.
Thirdly, the attack would not destroy Iran's uranium enrichment plants which are buried deep underground.
Israeli and American hawks claim that an attack could delay Iran's capability to enrich large quantities of uranium for three years, but Meir Dagan, former head of Israel's Mossad intelligence agency, thinks three months is optimistic.
Even if it were three years, Iran would be back to where it is now by 2015 - and an Iran that had been attacked by Israel and the US would be determined to get nuclear weapons as fast as possible. As Gen Martin Dempsey, the Chairman of the US Joint Chiefs of Staff, said recently, Israeli attacks on Iran "would be destabilising and would not achieve their long-term objectives".
If Mr Netanyahu and his fellow hawks truly believed that Iranian nuclear weapons would mean the extinction of the Jewish state, then their wish to attack Iran would be defensible, but they don't.
What's actually at stake here is not the survival of Israel, just the preservation of the huge strategic advantage Israel enjoys as the sole nuclear weapons state in the Middle East.
Ehud Barak, Israel's defence minister, let the cat out of the bag in a recent interview with Israeli journalist Ronen Bergman for the New York Times Magazine.
"From our point of view, a nuclear state offers an entirely different kind of protection to its proxies. Imagine if we enter another military confrontation with Hezbollah, (and a) nuclear Iran announces that an attack on Hezbollah is tantamount to an attack on Iran. We would not necessarily give up on it, but it would definitely restrict our range of operations."
Big deal. Israel lost its last military confrontation with Hezbollah in 2006 even with a monopoly of nuclear weapons, but it suffered no lasting harm as a result.
But there is also a deeper motive. Mr Netanyahu and his allies really think that an attack on Iran would bring the Islamic regime down.
As Mr Barak told Bergman: "An Iranian bomb would ensure the survival of the current regime, which otherwise would not make it to its 40th anniversary in light of the admiration that the young generation in Iran has displayed for the West. With a bomb, it would be very hard to budge the administration."
The thinking is delusional: the notion that the Iranian regime will collapse unless it gets the bomb is held by both Israeli and American hawks, but there is no concrete reason to believe it.
Gwynne Dyer is a London-based independent journalist.
