AND WHEN you go forth [to war] on earth, you will incur no sin by shortening your prayers127
if you have reason to fear that those who are bent on denying the truth might suddenly fall upon you:128
for, verily, those who deny the truth are your open foes.
Lit., "the prayer": a reference to the five obligatory daily prayers - at dawn, noon, afternoon, after sunset and late in the evening - which may be shortened and combined (the noon prayer with that of the afternoon, and the sunset prayer with that of the late evening) if one is travelling or in actual danger. While the extension of this permission to peaceful travel has been authorized by the Prophet's sunnah, the Qur'an mentions it only in connection with war situations; and this justifies the interpolation, in the opening sentence, of the words "to war". The prayer described in the next verse - with the congregation praying in shifts - is called salat al-khawf ("prayer in danger").
Lit., "might cause you an affliction" - implying, according to almost all the commentators, a sudden attack.