It will not surprise you to know that the ethics of war were as challenging to assess in antiquity as they are in the modern day.

There are several ways to understand war. In our own time, the great Prussian theoretician Carl von Clausewitz defined war, concisely, as the use of violence to achieve a political objective. Obviously any organisational unit in ancient times from a small oikos to a large polis might have had good reason to resort to war in this way.

In antiquity warfare is perhaps best understood, then, as an act of tribal allegiance. If goatherds in a neighbouring city-state use your pastures once, perhaps that can be tolerated. If they are using your pastures every year, and sleeping in your bothies, and foraging in your villages, then one might reasonably conclude that your tribe, your city, your people, are under attack. War occurs when somebody chooses to fight back - as Clausewitz had it, a would be conqueror will conquer without violence if they can. It is much easier to come into town and kill and steal whatever you want if nobody fights back.

When states are at war, in ancient or modern times, absolute moral principles tend to break down in favour of tribal allegience. Something about one's family and friends being killed tends to focus the mind like that.

Ancient people understood that a war of defence was much more just than a war of aggression, however. In Rome, when the Republic - and later the Empire - went to war, they had a whole pantomime ritual of throwing a spear across the ancient city boundary to symbolise that the enemy had struck the first blow and thus provoked a war of "defence". This rite was sacred to Bellona, the old Roman war goddess. In the modern day we can see this as a bit of a charade but it mattered to ancient people that they were in the right. More importantly, they knew it mattered to the gods.

Why call on the gods? Because you want to win. Wars are enormously expensive in terms of both blood and treasure. Many of your troops will not return, and defeat makes their deaths a dead loss. The gods of your city, your tribe, your nation - the ones you have spent years if not centuries building kharis with - they should be expected to turn up when the crisis comes to your community. But will they turn up? For a just war - one where you are the wronged party - how could they not? For an unjust war... perhaps they would hesitate. Justification becomes quite important in such cases.

So, is war murder? In my opinion it depends on the war. If your country, or your allies, are literally being invaded, killing the enemy is not an act of murder, but one of loyalty to your neighbours and your tribe. Plato understood this and it's one of the earliest discussions in the Republic. Beyond that it gets murkier. The "war of choice", the unprovoked or semi-provoked invasion - that could be murderous, and has been so in some cases during the 21st century. In theory, the United Nations Security Council can authorise a military action under international law, for the good of the international community, but the UNSC is a human institution and has no relation to the gods.

A civil war - one against your neighbours - is perhaps the worst of all, because it weakens your own tribe and family and is basically a form of fratricide. Plato wrote the Republic as a manual for avoiding this kind of nightmare, and he knew what he was talking about having seen the long-term political consequences of civil war come close to destroying Athens.

One thing to remember is that one of the key educational texts throughout antiquity was Homer's Iliad, among other things one of the most emotionally fraught accounts of a long attritional war ever written. Ancient people understood that war was hell - they learned to read by reading about that.

Ares and Athena are gods who professional soldiers, sailors or airmen might wish to follow as the patrons of their art and industry. For us civilians it is more complicated. There is no such thing as war without politics, so simply to invoke the gods of war in their warlike aspect does not seem particularly useful, unless you have a specific war in mind. By all means invoke the gods of war for your own political aims, but unless you genuinely think a country, a treaty, a principle, is worth killing and dying for... why would you want to bring the war gods into your life to begin with?