In Chapter 1, what had Bartle, Murph, and the other soldiers been doing for four days?

For four days Bartle, Murph, and the other soldiers had been crawling along rooftop grit. They slipped and slidded on a carpeting of loose brass castings left from the fighting the day before. They curled themselves into absurd shapes and huddled below the whitewashed walls of their position. They stayed awake on amphetamines and fear.