What is a peace & justice issue that's also related to philosophy?

The ethics of declaring war.

As it turns out, most things are philosophical issues. The most pertinent one that matches your criteria is probably just war theory (which discusses when is war morally justified), but there are other issues that concern peace and justice as well. One of the basic issues in political philosophy is why we ought to have a government in the first place. According to philosophers like Thomas Hobbes and John Locke, the primary reasons are precisely that civil society is more just and more peaceful than the state of nature (which is a technical term for the absence of government).

Regarding the latter issue, Hobbes thinks that human beings are naturally aggressive and warlike, and that only an organized society can control these impulses. Locke is more optimistic about human nature, but still thinks that the absence of a government more or less guarantees that our natural rights will be constantly violated (since no one has the authority to enforce them for us, and we can’t possibly protect ourselves from every potential violator on our own). This is a classic debate, and is closely tied to issues in game theory such as the prisoner’s dilemma.

If you’re looking for a topical issue, however, there are at least two that come to mind. The first concerns the legitimacy of borders. On the one side, you have the cosmopolitans who argue in favor of open borders. Nationalists, meanwhile, argue that certain groups have a right to create borders and keep others out. The other issue that’s popular right now is distributive justice, which is concerned with the distribution of material wealth (in the form of goods and/or money). The central debate here is what duty, if any, those with plentiful resources might have to those with few resources. And if there is such a duty, there’s also the question of how extensive it is. Do we just need to keep people out of crushing poverty, or ought we do what we can to help them thrive?