GMing a city defense

[guncrazy]

So after a few well placed catastrophic failures, the PCs and the city they reside in find themselves under imminent attack by the local warlord in the Texas area. He controls the region and subjugates the surrounding towns. The PCs defied him in not giving what he wanted and failed miserably to smooth things over angering the militia who is now going to make an example that this is what happens when you defy him.

How should I conduct this fight? Use D&D thinking meaning think big play small? only conduct PC combat and their tasks and depending on their successes or failures influences the overall defense? Some ideas would be most welcome