The ancients were quite concerned with the requirement for some sort of justification for a war, even though it was also well understood (as it is today), that the actual reasons for a war and the reasons given by politicians are not necessarily the same thing. I'll quote Polybius at length here, because he explains this phenomenon well and it still rings true today:

"Some historians of the Hannibalian war, when they wish to point out to us the causes of this contest between Rome and Carthage, allege first the siege of Saguntum by the Carthaginians, and, secondly, their breach of treaty by crossing the river called by the natives the Iber.

But though I should call these the first actions in the war, I cannot admit them to be its causes. One might just as well say that the crossing of Alexander the Great into Asia was the cause of the Persian war, and the descent of Antiochus upon Demetrias the cause of his war with Rome.

In neither would it be a probable or true statement. In the first case, this action of Alexander's could not be called the cause of a war, for which both he and his father Philip in his lifetime had made elaborate preparations: and in the second case, we know that the Aetolian league had done the same, with a view to a war with Rome, before Antiochus came upon the scene. Such definitions are only worthy of men who cannot distinguish between a first overt act and a cause or pretext; and who do not perceive that a cause is the first in a series of events of which such an overt act is the last. I shall therefore regard the first attempt to put into execution what had already been determined as a "beginning," but I shall look for "causes" in the motives which suggested such action and the policy which dictated it; for it is by these, and the calculations to which they give rise, that men are led to decide upon a particular line of conduct. The Soundness of this method will be proved by the following considerations. The true causes and origin of the invasion of Persia by Alexander are patent to everybody.

They were, first, the return march of the Greeks under Xenophon through the country from the upper Satrapies; in the course of which, though throughout Asia all the populations were hostile, not a single barbarian ventured to face them: secondly, the invasion of Asia by the Spartan king Agesilaus, in which, though he was obliged by troubles in Greece to return in the middle of his expedition without effecting his object, he yet found no resistance of any importance or adequacy. It was these circumstances which convinced Philip of the cowardice and inefficiency of the Persians; and comparing them with his own high state of efficiency for war, and that of his Macedonian subjects, and placing before his eyes the splendour of the rewards to be gained by such a war, and the popularity which it would bring him in Greece, he seized on the pretext of avenging the injuries done by Persia to Greece, and determined with great eagerness to undertake this war; and was in fact at the time of his death engaged in making every kind of preparation for it.

Here we have the cause and the pretext of the Persian war. Alexander's expedition into Asia was the first action in it." (Pol 3.6)

Polybius had a lot of fascination with this aspect of warfare, its causes and the excuses used to wage it with a facade of 'justice'. A very good case in point is the moral gymnastics the Romans made over deciding whether or not to go to war with Carthage in 264 BC, where they saw a good opportunity to intervene in Sicilian affairs and potentially stop the Carthaginians from expanding their power, but to give themselves a presentable excuse for intervening they had to support a group of mercenaries that the Romans themselves considered criminals in a dispute where they were pretty clearly in the wrong (Pol 1.6-10).

Polybius and other prominent commentators on such matters such as Thucydides seem to agree that this is fairly common conduct among nations. However, in comparing the ancient to the modern world is important to recognise that imperial expansion, or expansion of hegemony, was recognised as natural behaviour for states. Thucydides gives a particularly cynical view on the matter from the mouths of Athenian envoys trying to intimidate a smaller city-state:

" ‘For ourselves, we shall not trouble you with specious pretences—either of how we have a right to our empire because we overthrew the Mede, or are now attacking you because of wrong that you have done us—and make a long speech which would not be believed; and in return we hope that you, instead of thinking to influence us by saying that you did not join the Lacedaemonians, although their colonists, or that you have done us no wrong, will aim at what is feasible, holding in view the real sentiments of us both; since you know as well as we do that right, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must.' " (Thuc. 5.89)

This sort of conduct was natural to states in the ancient world, not only the great ones but also smaller ones. It was acceptable and normal for states to intimidate and attack their neighbours to expand their power (Eckstein 28, 315). Observers such as Polybius emphasise the real reasons for war as being due to simple yet powerful factors such as anger, fear and greed (100), and yet we can see in the ancient writers a clear emphasis on the necessity for public justifications before war could be waged. Much of this was to do with the belief in the importance of divine favour in allowing one to be victorious in war, which mean that a war had to be a just war. However, a 'just war' in Roman eyes could be justified by any challenge or even perceived challenge to honour and status (Mattern 171, 182-4), and according to Eckstein this was fairly typical for all powerful ancient states. There was still a line to be drawn however: note for instance that while Polybius analyses with relish Rome's reasons for going to war with Carthage in the first place, he is critical of the Roman decision to annex Corsica and Sardinia c. 238 BC on a flimsy pretext, capitalising blatantly on Carthage's weakness while fighting the Mercenary War (Pol 1.88). There were of course other 'rules', such as it being considered unjust to attack during any sort of truce (Livy 40.27.9-10).

So while there differences, we should also note that thanks to human nature, there are also similarities. It is argued by Lebow that even today, support is drummed up for wars with broad ideas of fear and national honour (8-9). He argues, compellingly in my view, that while states can in the modern day engage in warfare for various reasons, including security, economic interests and international standing (all powerful ancient considerations too), but that broadly "most if not all foreign policy behaviour" can be reduced to three main motives: fear, interest and honour. This interpretation is quite readily applied to most modern wars just as it could be to ancient wars. So while state violence for the purposes of protecting and expanding hegemony was far more 'accepted' (even if it was resisted tooth and nail) in the ancient world, I'd have to say that yes, there was a strong need for moral (and religious, though not so much 'diplomatic', as there was no real international diplomatic system) public justification for wars, just as there is today.

Sources:

Polybius, Thucydides: translations from Perseus.org

Livy, Loeb translation.

Eckstein, Arthur M., Mediterranean Anarchy, Interstate War, and the Rise of Rome, Los Angeles: University of California Press, 2006.

Lebow, Richard Ned. Why Nations Fight: Past and Future Motives for War, New York: Cambridge University Press, 2010.

Mattern, Susan P., Rome and the Enemy: Imperial Strategy in the Principate, Los Angeles: University of California Press, 1999.