Europa Universalis: IV. Happens often, always due to chain reactions occurring with alliances/wars especially now with Rights of Man expansion that added Great Power interventions.
Just this last weekend on Saturday I had planned to do something else for the day but in the morning I started up the game just to play for about 30 mins, turns out during that time my ally, Denmark would break the alliance with me (Commonwealth after forming it from Poland/Lithuania) whom was very important for me due to their navy, they broke it because their attitude changed to domineering and wanted to subjugate me, and Brandenburg would drag me into a war soon after (my other ally) where I am fighting on three fronts (due to various alliances being called in), the Ottomons to the south, Russia to the East, and a very strong Hungary to my west. Myself, Russia, and Ottomons were Great Powers, so another great power, France, did join in on my side as an intervention but it didn't help as much as I'd hope.
The war lasted for about 3 hours continuous gameplay, and we lost, but fortunately I did not take a big hit as I was expecting at first given my situation because I wasn't even prepared for the war, my military tech was behind, my manpower reserves were low, but I did have to cede the entire Crimean peninsula to the Ottomons as part of the peace treaty, even though about 50 years before this war I had taken it from the Ottomons in pretty much a near stalemate war too that put me into a lot of debt. Spent more hours playing rebuilding the strength of my nation/army because I was 100% sure the Ottomons would attack again and try and take more land from the south once our truce from the treaty expired. They did, but luckily I had won the war (whom I had to fight Russia once again on my east since they were allied), but I was able to force them to annul their alliance treaty in the peace deal which was really important because it would stop the Ottomon advance into Europe which Russia was helping. At this time I was the evening already, spent the whole day playing even though I had not planned to at all, wanted to do some other non-video game hobbies.
Granted pretty much every strategy game does this to me, particularly Paradox's.