Lothar turned out to be suited for military life, and rose quickly to be the Armsman of the Brotherhood of the Horse. Eventually, he was able to lead all of Azeroth's armies. But who was there to fight? The trolls? Everyone invades the troll lands. They're the Poland of Azeroth. And you can only kill so many kobolds and take so many candles before you accidentally burn down Stormwind.
But luckily for Lothar, an enemy conveniently arrived in the form of the orcs. They were ugly, green, and vicious, and no one knew where they came from. They quickly advanced to Stormwind Keep and attacked its walls. Lothar approached King Adamant Wrynn and told him that they needed to be aggressive in fighting the orcs. (As opposed to passive-aggressively mocking their conquering style?) Wrynn pledged to rid the land of the orcs, but died before he could do it. His son, Llane Wrynn, took up the fight and pushed the orcs back to Swamp of Sorrows.
However, treachery was afoot! A valuable artifact was stolen by a band of ogres hiding out in what is now the Deadmines. Lothar led an expedition to get it back, but was instead captured "to be killed slowly", according to WoWWiki. The ogres held him captive for twenty months, so it must have been really slowly. Death by a thousand papercuts slowly. Also, didn't the army realize that their commander was missing for those twenty months?
Anyway, he was eventually freed to rejoin the battle -- which, after twenty months without Lothar, wasn't going too well for Azeroth. The Horde was advancing. Even worse, Lothar learned that his old pal Medivh was responsible for summoning the orcs into Azeroth. Such betrayal could not stand, and Lothar led a force to kill the rogue wizard. Medivh's apprentice, Khadgar, stabbed Medivh through the heart while Lothar cut off his head. Hey, guys, he's not a vampire. But ... it was too late. There were too many orcs in the land. Garona assassinated Llane Wrynn, the orcs sacked Stormwind and put up a number of ugly tent-buildings, and Lothar led the survivors of Azeroth across the sea to Lordaeron.
Lothar presented his case to King Terenas Menethil of Lordaeron, who agreed that something had to be done about the orcs. Menethil brought the human kingdoms, the high elves, and the dwarves together to form the Alliance of Lordaeron. Since none of the human kingdoms wanted one of their rivals to be the leader of the Alliance, they decided to make the temporarily nationless Lothar their supreme commander. He appointed General Turalyon as his second-in-command, and Khadgar, Daelin Proudmoore, and Uther his lieutenants.
This combination of superstars proved to be unstoppable. They blazed a trail back down the continent, pushing the orcs further south every day. In a prediction of future PvP battles, the bulk of the Alliance and Horde armies met up at the foot of Blackrock Mountain (then a Horde base.) The batle was massive and chaotic, with each side taking heavy casualties. In the confusion, Lothar found himself dueling Orgrim Doomhammer, the Warchief of the Horde. Isn't that awesome? I think today's wars should be settled by the leaders whacking at each other with swords that are longer than their actual bodies.
Anyway, Lothar and Doomhammer dueled until both of them were exhausted. At that point, one of two things happened: a) Doomhammer shattered Lothar's sword, leaving him defenseless, and then smashed his skull on the next hit or b) some Horde rogues snuck up behind Lothar and bopped him on the head in a rather ungentlemanly fashion. You can guess which side takes which interpretation. Doomhammer thought that Lothar's death would end the battle. However, Turalyon picked up Lothar's sword and took over the battle, yelling "For Lothar!" The Alliance won the day and captured Doomhammer. Later on, Khadgar closed the Dark Portal, albeit temporarily.
Where he is now: Still dead.
His legacy: Lothar still has a presence in the hearts of the Alliance. A huge statue of Lothar in his last charge lies just west of the Pillar of Ash in the Burning Steppes. WoWWiki notes it was built by orc slave labor. It would be deliciously ironic if that included Orgrim Doomhammer. There's also a huge ugly monument to him in Honor Hold, as a memorial of Turalyon's man-crush on his mentor. And the survivors of the Alliance Expedition in Outland go by the name of the Sons of Lothar.
You can also grab one of Lothar's swords if you head to Blackwing Lair and defeat Nefarian. If you're lucky, you'll get Ashkandi, the Greatsword of the Brotherhood -- with A.L. inscribed on it. My guild had this drop three out of our first four Nefarian kills. We almost had to give it to a hunter. True story.
For more information: WoWWiki on Lothar, WoWWiki on the First and Second Wars, Nurfed on Ashkandi.