Political assassinations require considerable preparation and planning. It is almost certain the killing last week of Mohsen Fakhrizadeh, head of the Iranian nuclear program, was timed to avoid cancellation of the hostage exchange involving Australian-British academic Kylie Moore-Gilbert.
The nuclear program is under the control of the Iranian Revolutionary Guards Corps, and the Iranian terrorists released from Thailand had gone there to carry out an attack on the Israeli embassy following an earlier Israeli assassination of another Iranian nuclear scientist.
Australia would almost certainly have consulted Israel about the plan to exchange the prisoners for Kylie Moore-Gilbert. However, there is no reason to believe Australia would have had prior knowledge of the Israeli intention to carry out the assassination, if that is indeed the country behind the attack.
There are several possible reasons for the timing of the assassination. One obvious explanation is an intention to remind the IRGC that Israel will continue to extract a price from the IRGC for any attacks on Israel or Israelis, either directly or through surrogates including Hezbollah.
The risks of witnessing a spiral of military responses are not all that great, despite the rhetoric which flowed from Tehran after the attack.
Another perhaps secondary but still possible explanation is that the Israelis (with US President Donald Trump’s blessing) want to take every opportunity to damage the prospects of president-elect Joe Biden reaching agreement with the Iranians (in effect, the Supreme Leader and the IRGC) to resume US participation in the Joint Comprehensive Plan of Action (JCPOA), which is anathema to Israel, Saudi Arabia and Trump.
The IRGC was never keen on the JCPOA in the first place. To be seen (and internally, for the benign faces of the regime) to be complying with the US after a national humiliation of this nature would be doubly difficult.
On the other hand, a violent Iranian response against Saudi, Israeli or US targets would make it doubly difficult for Biden to pursue the idea of reviving the JCPOA as well.
In short, it would be a clever tactical move for the Israelis, at least in the short term. In the long term, well, that can be a long time coming.
The risks of witnessing a spiral of military responses are not all that great, despite the rhetoric which flowed from Tehran after the attack. Military options for the Iranians are few (the attack on Saudi oil facilities in Abqaiq in September 2019 produced nothing of value to either side, and an attack on US forces in Iraq, and anything more than symbolic reaction against Israel from Lebanon or Syria, would be just plain foolhardy).
The Iranians are capable of doing without the JCPOA. So-called “decapitation tactics” may or may not prevent the nuclear program proceeding (but if it proves unstoppable, it will probably be pursued without any further monitoring from the International Atomic Energy Agency if the JCPOA goes by the wayside).
The assassination places Biden in a very awkward position, between his promises of support for Israel and his supposed commitment to the rule of law in international affairs (although if you want an opinion on Iran and the rule of law, just ask Kylie Moore-Gilbert).