This story or novella probably dates from the '80s and I probably read it in Analog or Galaxy, but I cannot swear to it.
An exploratory team of humans, or maybe a small colonizing team, (call them "We", Our) has landed on an alien planet, with intelligent humanoid (?) inhabitants. The team establishes friendly relations with one group (tribe ? nation ?) of the aliens, who are primitive by our standards, but with ethics and morals similar to our own. (Call them Our Allies.) Our Allies are threatened by another group (tribe?) with whom, if I remember correctly, they have a long-standing rivalry. War is about to break out, and Our survival could be threatened if Our Allies are overrun.
The leader of the human team helps with battle strategy, but perhaps his biggest contribution is a rousing speech to Our Allies on the eve of the deciding battle. (Somehow, the human leader has become fluent in the native language.) The rousing speech is a translated combination of Henry V's speech to the English soldiers before the Battle of Agincourt on St. Crispin's Day as presented by Shakespeare, and Churchill's "We shall never surrender" speech early in WWII.
Fired up by the speech, Our Allies win the decisive battle, and.....???? I have now told you 100% of what I remember, except that it was a great story or perhaps novella.