An army suffers from non-combat losses. There is a 10% chance each time you move from one fief to another that you will lose some men, as well as for any days you have left at the end of the season. Attrition can wipe out your entire army if you enter a low population fief with a large army (over 60,000 troops). Armies had to live off the land in Medieval times and the troops would simply drift away when things got rough. Battles were easy, starvation was hard. Attrition rate varies by season with winter and spring being the times when losses are the greatest.