Commentary
Out of the verses appearing above, the first two (15-16) tell us about a military law of Islam. The word: زحف (zahf) translated here literally as 'marching to battle' denotes the confrontation and intermingling between the two armies. The sense is that once a war is on, turning back and deserting the battlefield is not permissible for Muslims.
The second verse (16) refers to an exception to this rule, and to a severe punishment for illegal deserters.