Jamieson, Fausset and Brown on Matthew 5:41

41. And whosoever shall compel thee to go a mile, go with him twain--an allusion, probably, to the practice of the Romans and some Eastern nations, who, when government despatches had to be forwarded, obliged the people not only to furnish horses and carriages, but to give personal attendance, often at great inconvenience, when required. But the thing here demanded is a readiness to submit to unreasonable demands of whatever kind, rather than raise quarrels, with all the evils resulting from them. What follows is a beautiful extension of this precept.