A republic can be a democracy internally and a conquering power externally. Rome, from which we get the word "republic," was a conquering power and this was the intention of the theoreticians who invented classical republicanism. Republics can and do come into conflict.
We need a new ideal of governance. It is not enough to insist that a polity be democratic internally; it must exist in a world order of polities which respect each other. I do not see how this is achievable without most of those polities being themselves democratic internally; a ruler who tyrannizes their own people will not hesitate to go conquering.