COMMENTARY on 4:102
Mustafa Khattab:

Translation:
When you ˹O Prophet˺ are ˹campaigning˺ with them and you lead them in prayer, let one group of them pray with you—while armed. When they prostrate themselves, let the other group stand guard behind them. Then the group that has not yet prayed will then join you in prayer—and let them be vigilant and armed.32 The disbelievers would wish to see you neglect your weapons and belongings, so they could launch a sweeping assault on you. But there is no blame if you lay aside your weapons when overcome by heavy rain or illness—but take precaution. Indeed, Allah has prepared a humiliating punishment for the disbelievers.
Commentary:
32   When the first group prays, the second group stands guard behind them, then the first group withdraws to the back when they finish their prayers and stand guard when the second group moves up to pray.

 

A. Yusuf Ali:

Translation:
When you (O Messenger. are with them, and standest to lead them in prayer, Let one party of them stand up (in prayer) with you, Taking their arms with them: When they finish their prostrations, let them Take their position in the rear. And let the other party come up which have not yet prayed - and let them pray with you, Taking all precaution, and bearing arms: the Unbelievers wish, if you were negligent of your arms and your baggage, to assault you in a single rush. 618 But there is no blame on you if you put away your arms because of the inconvenience of rain or because you are ill; but take (every) precaution for yourselves. For the Unbelievers Allah have prepared a humiliating punishment.
Commentary:

618  The congregational prayer in danger in face of the enemy rests on the principle that the congregation should be divided into two parties; one party prays while the other watches the enemy, and then the second party comes up to prayers while the first falls back to face the enemy; either party does only one or two Rak'ahs, or about half the congregational prayer; every precaution is taken to prevent a rush by the enemy; even while at prayers armour and arms need not be put off except when rain is likely to cause inconvenience to the wearer and damage to the arms, or when illness or fatigue causes the wearer's strength to fail. Details can he varied according to circumstances, as was actually done by the Prophet at different times.

 

Muhammad Asad:

Translation:
Thus, when thou art among the believers 129 and about to lead them in prayer, let [only] part of them stand up with thee, retaining their arms. Then, after they have finished their prayer, let them provide you cover 130 while another group, who have not yet prayed, shall come forward and pray with thee, being fully prepared against danger and retaining their arms: (for) those who are bent on denying the truth would love to see you oblivious of your arms and your equipment, so that they might fall upon you in a surprise attack. 131 But it shall not be wrong for you to lay down your arms [while you pray] if you are troubled by rain 132 or if you are ill; but [always] be fully prepared against danger. Verily, God has readied shameful suffering for all who deny the truth!
Commentary:
129  Lit., "among them". The "thou" in this sentence refers, primarily, to the Prophet and, by implication, to the leader of every group of believers at war with "those who deny the truth".
130  Lit., "when they have prostrated themselves, let them [i.e., the other group] be behind you". This idiomatic expression is not to be taken literally: in classical Arabic usage, the phrase kana min wara’ika (lit., "he was behind thee") signifies "he protected thee" or (in military parlance) "he covered thee", and is not meant to describe the physical relative position of the two persons or groups.
131  Lit., "turn upon you in one turning".
132  I.e., if there is a risk of their weapons being damaged by exposure to unfavourable weather conditions, the warriors are exempted from the obligation of keeping their arms with them while praying. This exemption applies, of course, only to such of the soldiers as are in charge of particularly sensitive weapons; and the same applies to the individual cases of illness mentioned in the sequence. It must, however, be remembered that the term matar (lit., "rain") is often used in the Qur’an to denote "an affliction": and if we adopt this meaning, the above phrase could be rendered as "if you suffer from an affliction"- thus allowing for a wide range of possible emergencies.