(To illustrate some of the issues.)
As the classic example of the effects of physics upon interstellar relations, consider the short and inconclusive conflict known as the Odeln Extrality Incident (from the Imperial perspective), or the First Border War (from the Republican perspective). It is generally agreed that the cause and first engagement of the conflict was the mutual destruction, a short distance outside the Odeln system volume, of the Imperial destroyer CS Joyful Dirge, and the Voniensan battlecruiser VNS Deliberative.
The causes of the incident remain obscure, beyond the rising post-contact tensions in the Seam, and even the details of it are disputed. Many of the ensuing skirmishes and inconclusive negotiations can be attributed to the relativistic nature of the engagement; the consensus of the Imperial observers, computed according to the empire time reference frame, was universally that Deliberative fired first upon Joyful Dirge, while the consensus of Voniensan observers in-system, operating in reference frames tied to Republic Universal, was that Joyful Dirge fired first upon Deliberative. It is the unfortunate nature of relativistic space-time that all of these observations can be simultaneously truthful and true.
Such are the difficulties of the strategos, the diplomat, and the historian in a non-Callaneth-compliant universe.