The First World War is a pretty good example actually, given that most of the nations got involved due to preexisting military alliances.
Well except that it didn't happen that way.
While it is true that in the period leading to the first world war, there were a number of treaties, agreements and conventions between great powers which formed the basis for the later coalitions. But these treaties were mostly either very loose agreements of mutual friendship or very specific treaties of intent, but i don't think any of them of rose to the level of an ironcald mutual assistance pact. Closest thing were probably the agreements between Germany and Austria as they stated that one power would support the other in case of an attack by another great power, but there were clear definition of what "support" actually entailed and that led to long and confusing negotiations following the July Crisis. The pact between Germany and Italy, who originally was the third wheel in the so called triple alliance, just stated that the two powers would support each other in case either of them were attacked by France, and as we saw Italy ended up joining the other side. The Franco-Russian pacts were quite similar they simply stated that the two countries would support each other in case one of them were attacked by a member of the Triple Alliance, but provided no support against any other nations, seen by the lack of french support during the Russo-Japanese war. Britain while having signed several agreements with both Russia and France had never actually promised support, but choose to support them anyway. On top of all of that in theory none of the treaties actually applied, as none of them dealt with an attack on/by Serbia.
Non specific alliances really wasn't something a Great Power would really agree to as it greatly shackled their foreign policy.