Did the Catholics or Orthodox suspend the Lenten fast during times of war? It would make sense for an enemy nation with knowledge of the Catholic or Orthodox systems of fasting to attack during the fasting seasons, when the faithful were not well fed. As a corollary, it would make sense for a Catholic or Orthodox nation to abstain from war during a fasting season.
Are there any recorded instances, or perhaps some Catholic canon law or Orthodox economy, regarding the suspension of Lenten or other fasts during times of war?
1917 Canon 1244 (1983 Canon 1244) says:


  It is only for the supreme ecclesiastical authority to establish, transfer, or abolish feast days and days of abstinence and fast common to the whole Church.


1983 Canon 1245 says:


  Without prejudice to the right of diocesan bishops mentioned in ⇒ can. 87, for a just cause and according to the prescripts of the diocesan bishop, a pastor can grant in individual cases a dispensation from the obligation of observing a feast day or a day of penance or can grant a commutation of the obligation into other pious works. A superior of a religious institute or society of apostolic life, if they are clerical and of pontifical right, can also do this in regard to his own subjects and others living in the house day and night.


Related to this is St. Thomas Aquinas's Summa Theologica II-II q. 40 a. 4 question "Whether it is lawful to fight on holy days?"