The Catholic church tried time and again to establish what they called "Gottesfrieden" (God's peace, treuga Dei) to stop various wars and conflicts. While most of the time that did not succeed completely, they did manage to establish certain rules like no warfare on Sundays, holidays, during certain religious times of the year, during a Crusade and so on. For the better part of the high and late Middle Ages these rules were to a large degree respected and contributed significantly towards limiting fighting between knights and lesser nobles. The exception to this would be Norman England, where "Treuga Dei" as a concept did not exist. So while they were not able to stop warfare altogether, they tried to restrict it at least. Source: Sorry, only have German Sources: H.Hoffmann, Gottesfriede und Treuga Dei (1964); Josef Fleckenstein: Rittertum (2002)