Template:PolicyCards (Civ6)
Discipline is a Military Policy Card in Civilization VI.
Strategy
This is the other starting choice for your initial Military Policy slot (unless you play as Germany, which has an extra Military slot and will be able to use both Discipline and Survey), and is invaluable for dealing with Barbarian raiders. The +5 
Strength provides real edge against unruly hordes invading one's land or blocking early expansion during the early game. You should use the Policy as soon as they start turning into a problem, unless you've been lucky enough to have close neighbors who deal with the Barbarians for you. Incidentally, Discipline does maintain some usefulness after the Ancient Era, though you usually have much better uses for your Military Policy slots at this point.
Civilopedia entry
Discipline has been necessary for victory since the first “armies” blundered into each other, with breaches of discipline punished by anything from demotion to death (in various unpleasant ways). At first, in the days of civilian militia, the rules were fairly simple … basically, follow orders, no matter what. With the rise of professional armies, military law became codified and more convoluted, although punishment hasn’t changed much – between 1942 and 1961 AD 160 members of the American military were executed under the auspices of the Uniform Code of Military Justice for various crimes, including desertion, mutiny, and murder.