Spoken by the Athenians and resulting in a war that, as Thucydides's audience knew quite well, Athens lost big-time.

Which actually holds up quite well for everybody who loves to bring up that quote: realism aka "we shouldn't face the consequences of our actions" is the obvious rallying cry for people facing the consequences of their actions.

It is descriptive, but not prescriptive.

If neither side can agree on peace, if neither side has objectives which the other will accept, if neither side is willing to compromise; What other outcome is possible in terms of realpolitik?

It is upsetting to observe. We all want better for humanity.

There have been cases in the past where an external strong power has been able to suppress both sides but it has to be done for generations until the reasons are lost to time.

Depending on who you ask, there have been a variety of external powers stirring the pot. Most people are horrified by the violence. Beyond the territorial, religious and cultural disputes there are opposing geopolitical factions.

Of course it is understandable to be outraged by the violence and atrocities. The human suffering is real, but arguments focusing on these points can miss the larger picture. The underlying incentives dictate outcomes. Atrocities are often marketed as rationalizations for further violence.

We want to prescribe an outcome without atrocities. Yet discussions fall into recrimination before they can describe the conflict coherently.

When power, not justice, guides actions and policies, yes.