Just asking if there was any record of Muslim soldiers fasting during a campaign, from the early years of Islam to the present day.
Did you have a specific period/region in mind?
A while ago u/CptBuck posted this answer about the Ottoman army in particular.
It is my understanding that the Qur'an makes specific fasting exemptions for travellers, soldiers on campaign and others, but I am no scholar of Islam and happy to be corrected on that point.
The 1973 war between Egypt and Israel occurred during Ramadan and some sources indicate that Muslim Egyptians fasted during the war.
From NPR
"The 1973 conflict that Israelis call the Yom Kippur War is known to Egyptians, Jordanians, and Syrians -- who launched their surprise attack while fasting -- as the Ramadan War."
From an interview with a Christian Egyptian veteran of the war from Euronews:
“Our Ramadan table is called the National Unity Media, and it’s open to everyone - Muslims, Christians, we don’t differentiate,” he explains. “As for me, I’ve been attached to the month of Ramadan since the October War [1973 Arab–Israeli War]. I was an officer, and we were fighting during Ramadan, and I could not not fast with my soldiers"
Sources:
https://www.npr.org/templates/story/story.php?storyId=129149577
https://www.euronews.com/2019/05/10/ramadan-unites-christians-and-muslims-in-egypt