Casus Belli or not...

[Metalstrm]

Sorry but it seems I only have questions to ask, and none to answer. Anyway, I was wondering how much was the concept of a casus belli a part of ancient warfare? Did all ancient people make the reasons for going to war a public knowledge, and did they base those reasons on justifiable (at least by modern standards) grounds, other than purely for honor or sadistic pleasure or both?

I know that fabrication of such reasons happened from time to time, but did they at least try to give the impression they were fighting for a justifiable reason?

[Marcus Julius]

I assume you aren't talking about the basic "He started it." concept of retaliating against an attack by the enemy.

The sale is easy, really. It was to bring the fruits of Roman civilization to an otherwise uncivilized world (and of course to bring to Roman civilization all of the natural resources that the uncivilized world possessed. Shhh, don't tell anyone). Much in the same way that there are now wars in the modern world to "bring democracy to an oppressed people".

The Greeks used the same idea prior to the Romans. It's always "because those who are to be conquered deserve to live the way that we do". It's been working for more than two millenia. Why change it?

Then, if that doesn't work, as a leader you could always fall back on "It is the will of the Gods! (or God, or Allah, or take your pick of any number of religions)".

[Tiberius Clodius Corvinus]

The Romans (if we think them for clarity as homogenous faction) fought only wars they considered just ;-) ) ... this mentions Mauricios in the (late Roman) Strategicon.
... at least this was their self-conception. In reality one might say they were not very keen to avoid wars and found a reason to justify what they did, whenever they needed a cause.
But otherwise it's always problematic to apply modern ideas of morale and humanity. Honor was an very important part of ancient society. Its moral concept was not to divide between good and bad in our sense, but more between dishonorable and honorful.

[Carlton Bach]

Roman law was very keen on the idea of a just war. You could not just invade someone, you always needed a reason. Of course, reasons were plentiful, but still, you will find that especially during the Republic, much effort was expended on creating a situation in which the enemy 'started it'. If your political enemies could show your war was unjustified, you might - theoretically - find yourself extradited (some senators wamnted to do this to Caesar).

[Robert Vermaat]

The Romans (if we think them for clarity as homogenous faction) fought only wars they considered just ;-) ) ... this mentions Mauricios in the (late Roman) Strategicon.

Actually, this is a maxim, Maurikios advises that only 'just' battles the should be fought. 'Unjust' wars were considered bad for moral and therefore harder to fight.

[Tiberius Clodius Corvinus]

There were certain rules which had to be followed before a war could be started. So there was a dateline given to the enemy to settle the matter, before the Romans did attack. (Of course the Romans demanded sometimes things they were sure the others could never agree with.)
I'd add that the Romans were really creative in shaping situations so that they did fit into their law.
A "funny" story I heard in a lecture. Rome was at war with, I think Syracuse. To go to war it was a necessary (religious ?) act that a Roman priest threw a javelin in Syracusian territory, which was to far away. So the Romans forced a captured Syracusian soldier to buy a piece of land in Rome and threw their javelin in that piece of land. So much on law ... ;-) )

...some further quotes on Roman attitude towards law:

"Cease quoting the law. We carry weapons!" by Pompey the Great (to a city which objected to being besieged)

"If you must break the law, do it to seize power, in all other cases observe it." by Julius Caesar