The revelation of an Iranian plot to kill a Saudi diplomat on American soil was not particularly surprising. The U.S. has been engaged in a low-level war with Iran for years, although the war appears to be pretty one-sided, with Iran actively working to kill American soldiers, and doing everything possible to create anti-American proxies in Iraq -- among other things. Iran has been allowed to get away with this, despite its military weakness compared to the United States. Now it is true that there have been covert actions against Iran too, such as the killing and kidnapping of nuclear scientists, and cyberattacks aimed at its nuclear program. The level of U.S. involvement is unclear, but it is likely that U.S. allies such as Israel or Saudi Arabia might be behind them. Overall though, the U.S. has tolerated open hostility and acts of war from a third-rate power.
Some experts are surprised and skeptical about this plot's links to the Iranian government, because it seems reckless and somewhat amateurish. They could be right, and it might have been some sort of rogue operation. But I doubt it. I find it unsurprising that Iran might engage in what seems like a reckless escalation of the war, because it has little reason to fear any sort of significant response. It is well aware that the U.S. is preoccupied with various major issues, and that it is led by an extremely weak president, who has been nothing but spineless toward all enemies other than Al Qaeda and its affiliates. You have only to look at Obama's weak response to this situation. The U.S. government openly accuses Iran of trying to assassinate an allied diplomat on American soil, but then does nothing other than run to the UN and try to get more useless sanctions. Why should the Iranian rulers fear that sort of toothless action?
This situation demonstrates yet again that the Obama administration is incompetent when it comes to foreign policy. You don't accuse a hostile country of an act of war unless you are prepared to respond with force. A strong accusation combined with a weak response does nothing other than increase hostility while making you appear weak. One thing Obama is very good at is making the U.S. look weak.