The most famous example is Custer's Last Stand, where a U.S. military platoon was attacked by the Apache tribe in retaliation for the U.S.'s repeated acts of violence and oppression against them and their fellow tribes. Custer's men were all geared with military-grade rifles, pistols, and cannons, while the Apaches had only axes, knives, and bows. It certainly helps that the Apache had the numbers advantage, too, but it was an example of a lesser-geared fighting force prevailing over a contemporary military entity.
Alternatively, there was a famous British general Jack Churchill (also called Mad Jack), who fought during World War II with a bow and arrow, a set of bagpipes, and a Scottish claymore sword. He entered the war in 1940, where he constantly fought the Nazis, who were known for having the highest of high-tech gear of that time period. Jack never lost a battle, and it took an entire squadron of German assaulting him and six other men to capture him in 1944. And it was only because he was knocked out. By a grenade. And the Allies' advance scared his German captors, and they all ran away, leaving their prisoners behind. Jack promptly got up, walked about 100 miles to Verona, Italy, and decided the European Front was too easy, so he fought in the Pacific Front until the war ended. He then spent the next 15 years teaching at various military academies, and he lived until 1996. Despite fighting the Nazis with a sword.