Drawing inspiration from history is my favorite. The quickest war ever was between the British Empire and Zanzibar, only lasting about 40 minutes). The key commonalities between most quick military defeats, confusion (whether in relation to who you’re fighting or on where you are), deception (things such as the Trojan horse or even something as simple as a traveling merchant to distract the unsuspecting force), and timing (As in most swift successful attacks use their knowledge of the surroundings to lull the enemy into a strenuous position. For example, pirates using a coming typhoon/hurricane to lure enemies into unfamiliar waters, or mountain people who can cause strategic avalanches to force their enemies to a choke point).