Oh, three more comments:
First comment: Lead By Example is triggered by reducing an enemy to 0 HP or by dealing maximum weapon damage. Tying something to maximum weapon damage is kind of odd in several ways:
1.) Most 5E specials like Swords of Lifestealing trigger off of crits, not maximum damage, so it's breaking the idiom.
2.) You'd wind up being three times as inspiring with a greataxe as a greatsword, since d12 maxes out 1/12 of the time and 2d6 maxes out 1/36, unless GWF is in play and then it's slightly more often.
3.) Unarmed strikes would be inspiring every single time, since they have a fixed damage of 1.
So it's probably better to just tie it to crits or reducing an enemy to 0 HP. And in fact, since (N)PCs sometimes feign death in order to escape the real thing, it should also trigger on apparently reducing an enemy to 0 HP too--since it's a morale effect and not a physical/mystical effect, how would you troops know if the enemy is faking it?
Second comment, you can tell that this is a good class because I immediately want to roll up a goblinoid fighter with this subclass and make him one of the villains/major NPCs in my campaign. Up until now the niche has been filled by Eldritch Knights and Battlemasters. So far it looks like Battlemasters are the cunning tacticians with brains, Warlords are lead-by-example types with force of personality, and Eldritch Knights are the personal combat champions with magic a la Death Knights. (Action Surged Fireball should terrify NPCs.)
Third comment: maybe the 18th level ability should not apply to the warlord's own attacks, since how can you inspire yourself? Then again, the Oathbreaker paladin does affect himself so it might be fine. Although the Oathbreaker's aura is mystical, so the Oathbreaker isn't just believing his own rhetoric, he's actually tapping into a physical phenomenon that affects himself as well as his undead. I'm partially afraid that if a 20th level Warlord is doing 4 attacks at +5 to damage for each attack, that might overshadow the Champion and the Battlemaster. That can't happen with the Oathbreaker because he doesn't get 4 attacks. I go back and forth on this, but if I roll up any NPCs with this class I think I will not be letting him affect himself with this, any more than the Lore Bard can give himself bardic inspiration or the wild sorcerer Bend Luck on himself. It's strictly an other-person ability. Additionally, you might want to restrict the allies who can benefit somehow, by giving it specific fluff, so that DMs know whether or not Devastating Offensive should apply to skeletal archers, hordes (or swarms) of rats, and golems. Either restrict it (like Inspiring Leader) to those who can understand your language if it's intended to represent battle commands, or restrict it to those who are not immune to the charmed condition if it's supposed to be a lead-by-example thing. You could even let individual warlords pick which restriction applies, based on personal style, when they acquire the ability.
) welcomed.
