The enemy forces have penetrated deep into the heartland.
Tips
memory
腹 means 'belly/abdomen', 地 means 'land' - so 腹地 is literally 'belly-land', the inner core of a region (as opposed to the coast or border). The same metaphor exists in English 'heartland'.
usage
Used in geography (港口腹地 a port's hinterland), economics (经济腹地 economic interior), and military contexts (深入腹地 penetrate the heartland).
