The conflict in Syria is a very different one from that of the First World War, the centenary of which is now just a few months away.

But the poet Edward Thomas, writing 98 years ago in the context of that slaughter, had some prescient words which we do well to remember as we think about the situation in Syria today.

In one of his most famous compositions, he begins with the simple assertion: "This is no case of petty right or wrong, That politicians or philosophers can judge."

The poem came to mind as the BBC reported that David Cameron had "told MPs that the government will reserve the right to arm rebels in Syria without holding a vote in the House of Commons".

And it did so because the conflict in Syria is not a matter of black and white, of tyranny against liberty, of "good guys against bad guys". It is messy, complex, and multi-layered. And sending arms to any of the groups involved could well be as wise as throwing a match into a box of fireworks, hoping that only one particular rocket will ignite – and that once lit, it will travel in a specific, pre-determined direction.

The philosopher and theologian Thomas Aquinas (1225-1274) famously decreed that for a war to be just it must firstly be for a good and just purpose rather than self-interest; secondly that it must be waged by a proper authority such as the state; and finally that peace must be a central aim.

Just War Theory has endured over the centuries, and indeed been refined. As recently as 1992, the Catechism of the Roman Catholic Church lists four conditions for "legitimate defence by military force". These are that the damage caused by an aggressor must be "lasting, grave and certain"; that all other ways of ending conflict have been "impractical or ineffective"; that there are "serious prospects of success"; and that "the use of arms must not produce evils and disorders graver than the evil to be eliminated".

Even on a generous interpretation of these criteria, it is hard to see how arming particular groups of rebels in Syria would fulfil them. Moreover, even comparatively recent history should teach our leaders that armed intervention, however well-intentioned, can take on a life of its own in all sorts of unforeseen ways.

The Barnabas Fund aid agency has just this week highlighted the fact that independent United Nations investigators are expressing concern about over-simplification of the conflict. It quotes UN worker Paulo Pinheiro – who leads a team of experts documenting Syrian atrocities – as saying: "It was said the rebels were angels, but ... the majority of rebels are very far from having democratic thoughts and have other aspirations." He added: "There is a very complicated distinction between the bad and the good rebels."

Indeed, any distinctions at all are difficult. As the historian Tim Stanley put it in The Daily Telegraph, the choice between the Assad regime and any likely rebel alternative boils down to "choosing between two versions of medieval tyranny... Faced with this false choice, any kind of Western military intervention is morally compromised before it has already begun".

Although out of context, some further words from Edward Thomas's poem are nonetheless an apt conclusion here: "I have not to choose between the two, or between justice and injustice. Dinned with war and argument, I read no more."

But what we can do is to pray: after all, who could have foreseen the results that fervent prayer brought to Northern Ireland, South Africa and Eastern Europe? And we can do that now.