The goal is to find examples that don't imply that inspiration and/or tactical ability are inevitably linked to being in a position of authority over the rest of the group.
Aragorn was the acknowledged leader of the Fellowship after Gandalf's fall.
I don't think that being invested with authority is sufficient to establish a character as inspiring to his/her fellows. The history of the world is full of formal leaders whose underlings followed their orders out of simply duty rather than love or devotion.
And the reverse is also true (and is probably more common in fiction than reality): that an inspiring person, in virtue of the respect/devotion that s/he evokes in his/her fellows, becomes acknowledged as a leader. This is the case with Aragorn, I think.
Thus, I think you are reversing cause and effect. The reason that Aragorn was the acknowledged leader is because of his ability to lead, his capacity to inspire his comrades.
Here is what I think is the key passage (p 349 of the Unwin one volume edition, "The Bridge of Khazad-Dum"):
'He cannot stand alone!' cried Aragorn suddenly and rang back along the bridge. 'Elendil!' he shouted. 'I am with you, Gandalf!'
'Gondor!' cried Boromir and leaped after him.
At that moment Gandalf lifted his staff, and crying aloud he smote the bridge before him. . . . With a terrible cry the Balrog fell forward . . . But even as it fell it swung its whip, and the thongs lashed and curled about the wizard's knees, dragging him to the brink. He staggered and fell, grasped vainly at the stone, and slid into the abyss. 'Fly, you fools!' he cried, and was gone. . . .
Even as Aragorn and Boromir came flying back, the rest of the bridge cracked and fell. With a cry, Aragorn roused them.
'Come! I will lead you now!' he called. 'We must obey his last command. Follow me!'
They stumbled wildly up the great stairs beyond the door. Aragorn leading, Boromir at the rear.
We can see that Aragorn is able to inspire Boromir - his notional peer, as another "Captain of the West" - to follow him to Gandalf's defence. And we see that Aragorn assumes the mantle of leadership without any formal investiture of authority - and Boromir accepts it.
Later on, Frodo and Faramir discuss the nature of Aragorn's leadership of Boromir ("The Window on the West", p 696):
'. . . [A]lways it displeased [Boromir] that his father was not king. "How many hundreds of years needs it to make a steward a king, if the king returns not?" he asked. "Few years, maybe, in other places of less royalty," my father answered. "In Gondor ten thousand years would not suffice." Alas! poor Boromir. Does that not tell you something of him?'
'It does,' said Frodo. 'Yet he always treated Aragorn with honour.'
'I doubt it not,' said Faramir. 'If he were satisfied of Aragorn's claim, as you way, he would greatly reverence him. . . .'
But how did Boromir become satisfied of Aragorn's claim? At first, he was not ("The Council of Elrond", pp 263-4):
[Aragorn] cast his sword upon the table that stood before Elrond, and the blade was in two pieces. 'Here is the Sword that was Broken!' he said.
'And who are you, and what have you to do with Minas Tirith?' asked Boromir, looking in wonder at the lean face of the Ranger and his weather-stained cloak.
'He is Aragorn son of Arathorn . . . descended through many fathers from Isildur Elendil's son . . .'
Boromir's eyes glinted as he gazed at the golden thing. 'The Halfling!' he muttered. 'Is the doom of Minas Tirith come at last? But why then should we seek a broken sword?'
'The words were not the doom of Minas Tirith,' said Aragorn. 'But doom and great deeds are indeed at hand. For the Sword that was Broken is the Swrod of Elendil . . . Do you wish for the house of Elendil to return to the Land of Gondor?'
'I was not sent to beg an boon, but to seek only the meaning of a riddle,' answered Boromir proudly. 'Yet we are hard pressed, and the Sword of Elendil would be a help beyond our hop - if such a thing could indeed return out of the shadows of the past.' He looked again at Aragorn, and doubt was in his eyes.
It seems to me that it is Aragorn's valour, together with his inspiring personality, that satisfies Boromir of Aragorn's claim.
In 4e, PC development has a certain trajectory that makes this LotR-style character development easier to bring out in the game. Because of the transition from heroic tier to a paragon path to an epic destiny, a warlord PC's destiny of being a great ruler or leader can emerge as the game unfolds: eg at 11th level the warlord PC becomes a Knight Commander, retrospectively vindicating his/her earlier claims to be a leader; and then at 21st level s/he becomes a Legendary Sovereign.
I think that 5e doesn't build in quite the same trajectory for PC development, so there is perhaps a greater risk of a 20th level warlord not having progressed much beyond 1st level in terms of character backstory revealing how and why the character is an inspiring leader. But in a game in which the participants care about the story of the characters, I think they will be able to fill this in in an ad hoc way, even if the game's mechanics don't deliver it "automatically" in the way that they do in 4e.