I know that you are exempt from fasting during Ramadan if a war is on (and also for other various reasons). But - has it ever been customary (or even - required) for Muslim fighting forces to offer truces, or to refrain from going on the offensive, during the month of Ramadan?
No there has been no such obligation or custom. Muslims have fought plenty of wars against each others and non-Muslims in the month of Ramadan without any considerations of offering a truce just because it's that month.
Muslim vs Muslims
- Caliph Ali was murdered by a Kharjite on 21st of Ramadan. Granted it was act of one team instead of a proper battle but it is a part of the First Fitna (First Muslim civil war) and War against Kharjites. The first Fitna itself raged for years without any regard for Ramadan. Case of other Muslim civil wars is similar.
Muslims vs Non-Muslims
- The first battle of Islam, Badr was fought on 17th Ramadan, 2 AH.
- Second expedition of Wadi al-Qura was fought in Ramadan of 6 AH.
- Mecca was conquered by Muslims on 20th Ramadan 8 AH.
- Muslims won the decisive battle of Guadalete on 28-29th of Ramadan 92 AH.
- Muslims finally stopped the Mongol tide at Ain Jalut on 26th of Ramadan 658 AH.
No there is no instance of Muslims offering a truce just because the arising conflict happens to coincide with Ramadan. There is no provision for it in Muslim theology nor in their history.