War----or maybe not

So, for the first time another Civ declared war on me. The Arceans (even though I didn't think our relations were that bad. In fact I know they weren't).  I was surprised because while the relations were negative, they weren't really, really bad.

I am even more surprised now since they have not sent a single ship against me.  Even though I have surveyors flying all through their area grabbing anomalies.  I haven't seen a single ship from them.  They are listed as the strongest militarily.