What was the situation regarding ammunition for the invaders?

The chosen answer accurately captures the essence of the situation as it suggests a significant deficiency in resources, specifically ammunition, faced by the invaders. By stating "they were short of ammunition," this option clearly indicates a lack of sufficient supplies necessary for their operations.

This phrasing emphasizes the critical nature of their predicament, suggesting that the inadequacy of ammunition could impact their effectiveness in the invasion. In contrast, while other options might imply varying degrees of ammunition availability, none convey the specific and pressing shortfall that is central to understanding the challenges encountered by the invaders in this scenario.