I had a game a few weeks ago, started on monarch (v40.1), increasing difficulty (I manually let this adjust every 125 turns). My Ottoman AI neighbor, went early on full unit building, had a stack of about 30 units (1 city only), when my total units where less than 15, and all scattered around my borders and in my 2 cities. The stack reached my border (obviously with hostile intentions), I had previously scouted him and started building defensive units, moving the rest near my closest to him city. And then, reaching my border, the Ottomans just went back (around emperor or deity difficculty at that time).
Next they went to its other neighbor (hostile stack even larger), Elizabeth. Did the same exact thing. For some reason, it cancelled its plans for war each time, don't know why.
Eventually they started a war with me (having 3 cities, me 5-6 or more), I razed one of theirs, so that threw them a lot behind in score and power, and gave me additional expanding space.
Later on, me and my neighbor gal Elizabeth again on war with Ottomans, the Ottomans far back in tech than me and Liz, we wrecked them, but she never took any city out of him. The last Ottoman city, being more than 50% English and 0% my culture, I left it for Liz, who had a massive stack of about 40 units, including siege weapons. She never attacked it, but instead kept moving around it. I took my siege units, bombarded the city to 30% defense (the minimum at that era), took out most of the defender units...still Liz was just strolling around with her massive stack (I guess the country was appealing for her army?). So ... I just took the city and gave it to her as a present, some hundred years later gaining it back through culture revolt.
Oh I'm so waiting for the upcoming unit AI redesign! (Yes I am being patient Thunderbrd, just sharing my recent story about AI and its current art of war...)