Entrenchment *IS* an expansion pack. Effectively, Ironclad chopped the 'full' expansion into three parts, priced at 1/3rd each.
Frankly, the $10 Entrenchment expansion has more content already than most full $30 ones. It's honestly one of the best real deals I've seen.
Think of it as 'episodic expansion', except unlike Valve, they're not ripping you off completely while wasting their time with a three year wait between each one 
