I was playing as the Scots in the XL mod and took the next province south (whatever it's called) and the English king and his 2 sons came in as reinforcements. After I killed them all in that battle it was nothing but rebels to finish taking all of England. This happened on about the 5th turn. Then when I invaded France and took Flanders, then French had 2 civil wars about 3 years apart and basically ended up only owning Toulouse and I took over all the other rebel provinces.

The rest of that campaign was too easy and I soon switched to an Armenian one where almost the same thing happened and I killed the Egyptian king and his sons when I took Tripoli. The rest of Egyptian lands went rebel and I cut off the Turks ability to partake in the spoils and was quite large quite soon.

Then I started as the Portugese, that campaign was hard for almost 100 years as nobody was easy pickings.