Don't you mean the 12 nautical mile, ~14 statute mile, or 22 kilometer limit?
Nonsense, getting YOU to declare war is the entire point of the exercise. In diplomatic terms, it makes you the aggressor and various treaty agreements are enforced differently if you declare war on him rather than he on you.
And yet in the real world it isn't usually accepted for a nation to claim the waters around some foreign nation as their own territorial waters, and in fact making such a claim would be considered at the very least provocative. Nor does the 12 mile limit apply universally to all tracts of water; nations with regions of coastline which have regions of coastline which would create regions of overlapping territorial waters typically dispute the region of overlap or settle the issue by agreeing upon a treaty, and while a warship sailing in disputed waters is a provocative act, it is not an act of war. Sinking that warship, however, is an act of war, and while I would agree that the sinking was not unprovoked I would still count the side which sank the ship to be the aggressor, as they escalated the conflict beyond just rattling sabres.
This is in contrast to Galactic Civilizations II, in which it is not only accepted but even normal for spheres of influence to fully encompass foreign territory, and now we're told that spheres of influence to one degree or another represent a territorial claim by a nation - territorial claims which can be recognized and accepted by the international community, as shown by the United Planets laws which require nations to pay rent on their colonies and starbases which do not fall within their own sphere of influence. This is, for all intents and purposes, legalized (and legally mandated) extortion, especially in the case where the sphere of influence did not originally contain the foreign colonies but has since expanded to include them for one reason or another. I can accept disputes over who gets control over recently-settled regions where the colonies of various factions are intermingled, but I do not consider it as anything but naked aggression for one nation to make a claim to having territorial rights to another nation's well-established colonies in regions which did not fall within the claimant's sphere of influence prior to the establishment of these colonies. Yet the fact is that the United Planets will not only recognize these claims but also provides tacit support for them, as evidenced by the laws imposing a tax upon the colony owner.
Laying claim to a region of space which includes preexisting foreign colonies is a much larger act of aggression than the owner of those colonies "violating your space" in the region around those colonies with warships. Blowing up those warships which are "violating" your space in that manner is such an open act of aggression that trying to defend it is utterly ridiculous, and the idea that a faction should suffer diplomatic repercussions for declaring war in response to such an act is nonsensical. It is a somewhat more defendable position to take when the colonies in question were established in a region after you had laid claim to the region, though this is still rather debatable given that there is no concept of claim history or disputed claims within Galactic Civilizations aside from having colonies within the sphere of influence of another faction, as if I lay claim to a region and then you make claim for the same region somewhat later on, and then I establish colonies or build starbases within that region, or deploy warships to the region, this is an escalation of a dispute that you caused rather than an act of aggression on my part. The act of aggression was you claiming an area that I had already laid claim to, not my response to you making that claim. You may consider my response provocative, but that doesn't make me the aggressor. Galactic Civilizations fails rather badly when it comes to recognizing the existence of competing claims, however, and only recognizes the claim of the faction with the greatest influence in the region, whether the difference in influence strength is 1 influence or 1000, and does not track claim history except, to a degree, for planets controlled by a faction.
