The real world is never wholly predictable. I doubt if the Spanish allies of Napoleon expected him to put his brother on their throne, and push them into alliance with England and Portugal. It may have come as a surprise to Germany and Austria-Hungary when their ally Italy not only failed to support them in WWI, but declared war on them instead.

I like a bit of Machiavellian randomness in my diplomacy. At times it is agonizing when you are overstretched and a very friendly ally suddenly turns on you. It happened to me once in a GB campaign when France started a war very early and my closest ally UP stabbed me in the back. A frantic scramble for trade route survival ensued, but in the long run it gave me the opportunity to grab all of the Dutch colonies and trade spots in revenge, and sell a lot of dismasted fluyts!