Blood & Guts 2.0: Battlefield Unit Combat System

From Introduction:

The d20 System is an excellent system for handling combat engagements involving less than twenty people. However, what does the GM do when a combat situation involves hundreds of people? There are a number of strategies that wily GMs have adopted over the years to handle this, usually involving just a tight focus on the PCs and their actions during a larger battle. However, with the Battlefield Unit Combat System (BUCS), the GM now has the tools to run this size of battle like any other battle.