William Burkitt on Luke 3:14

Observe here, what a general resort there was of all sorts of persons to John's ministry; Pharisees, Sadducees, publicans, soldiers; these last here enquire of him what they should do to gain acceptance with God? He answers, Do no violence, defraud no man of his own by false accusation, but be content with the allowance assigned you for your maintenance.

Where it is, 1. Strongly supposed that soldiers are insolent oppressors, making no conscience or injustice, false accusation, and violent oppression.

Yet, 2. The office and employment of a soldier is not condemned, but regulated; he does not bid them cast away their arms, abandon war, appear no more as military men in the field; but manage their employment inoffensively.

Whence we learn, that in some cases, and under some circumstances, for Christians to make war is both lawful and necessary. To make a war lawful, there is required a lawful authority, a righteous cause, an honourable aim and intention, and a just and righteous manner of prosecution, without vanity and ostentation, without cruelty and oppression. Courage and compassion on the one hand, and cowardice and cruelty on the other hand, do frequently accompany one another.