One of the phrases trending on Twitter over the last few days has been '#FranzFerdinand' – and it's not in relation to the Scottish rock band which bears that name.

The reference is to Archduke Franz Ferdinand, whose assassination in June 1914 sparked a chain of events which led to the First World War. And it's gone viral because of fears that the assassination of Qasem Soleimani in Iraq could likewise unleash an unstoppable series of decisions which result in some terrible future conflagration.

Think about it: Donald Trump has mentioned 52 possible targets that could be hit depending on how Iran responds to Soleimani's killing – 52 being the number of American diplomats caught up in the 1979 Tehran embassy siege. In turn, Iranian President Rouhani has tweeted that Trump should 'remember the number 290' – a reference to 290 passengers killed when the US accidentally shot down Iran Air Flight 655 over the Gulf in 1988. And thus 'so it goes' – to use the repeated phrase in Kurt Vonnegut's powerful anti-war book Slaughterhouse Five.

It's hard to believe more than a few people outside the White House and Trump's circle of advisers believe the US president's threats are anything other than insane. The rest of the world watches on with horror, incredulity and fear as this man with the temperament of 'a 12-year-old in charge of an air traffic control tower' (as one book by a former Trump aide described him) demonstrates his ongoing unfitness for office.

But if there are any sensible people left in the White House ,they might want to dust off what is known as 'Just War Theory' and present a summary (ok, like a really simple summary) that Trump might be able to grasp before it is too late. Just War Theory has roots in Christian thought but in other traditions too. Broadly speaking, as one writer summarises it, 'a war must be in the service of a just cause, must have been commissioned by a legitimate authority and must be waged for a right intention. War has to have been the last resort for keeping peace, should have a high probability of success, and cannot cause harm disproportionate to the amount of good it is expected to achieve'. Of course, there is much, much more to be said. But that is the idea in a nutshell.

On this basis, many people today would not regard the First World War as a just war. But, by contrast, it would usually be argued that World War Two was a classic case of an absolutely just war. Again, the 2003 invasion of Iraq would not meet the theory's criteria. But the war against Islamic State certainly would, at least in my opinion.

How does Just War Theory shape our understanding of the current tension between the US and Iran? Of course, Soleimani was a man responsible for much destabilisation and violence in the Middle East. But it's not clear that Soleimani's assassination meets the criteria involved in a Just War. It doesn't seem to have been a last resort and looks likely to cause disproportionate harm in relation to any good achieved. Moreover, talk of the General being involved in some kind of 'imminent' attack on US troops has been both vague and unsubstantiated. And his replacement in his role is cut from the same cloth, so the same destabilisation and violence will continue regardless.

What's certainly true is that Trump's threatened retaliation in the event of future Iranian action doesn't even begin to meet the criteria of a Just War. Apart from anything else, he himself has declared that such a response could be 'in a disproportionate matter'. And legal experts reject Trump's assertion that notifying Congress by Twitter of his intentions in any way fulfils the War Powers Resolution regulating the involvement of the US in conflicts.

Furthermore, another part of Just War Theory is concerned with acting rightly once a conflict has started. Trump's assertions that some of his 52 selected Iranian targets might include 'cultural sites' clearly breaches that aspect of the theory, not to mention being prohibited by international conventions signed in Geneva and at the Hague.

Of course, there will be some who believe that invoking any concept of a Just War – especially as it is based at least partly on Christian principles – is outmoded. As someone said recently: 'There are those who say... sacred beliefs are outdated. But we know they are just the opposite. Our traditions and our values are timeless and immortal.' And the person who said those words? Er, step forward President Donald Trump...

Already more than 50 people have died as an indirect result of Soleimani's assassination – those killed in the crush at his funeral in Iran this week, with over 200 injured. Blood begets blood. Violence generates violence. A just war is a rare war. We do not have to be pacifists or even Christians to take to heart Jesus' words: 'Blessed are the peace-makers.' 

David Baker is a former daily newspaper journalist now working as an Anglican minister in Sussex, England. Find him on Twitter @Baker_David_A. A shorter version of this article appears in the new edition of Evangelicals Now.