Okay, so I have a continental truce with another guy. I have continent A, he has continent B. I couldn't invade him across the border, so I took a path that took me through continents C and D before I broke his bonus. No problems so far.
I took continent B with my next attack (the same one that went A->C->D->B). Still no problems. His beef, however, was that I fortified my troops from continent A into continent B, now that I took it. I have never attacked continent B through continent A, but I fortified across it. Are fortifications generally considered part of the "truce"?