To get warlords, you need: A failed state, a large (but decentralized) army, and a resource economy that can efficiently be made (by a warlord) to trade goods for arms (usually from overseas, but gigantic pre-existing arms stocks a la Iraq or the Western Roman Empire will do in a pinch). It helps if the failed state was autocratic and expansionist; that gives you lots of local garrison armies and simmering rebellions. Warlordism can resemble (or grow out of) degenerate feudalism, but pre-existing feudalism isn't a sine qua non.
This, I reiterate, is not an AH as we normally see it. It's a possible sketch map toward a solution: Pulp Era American Warlords. If we can get there some other way in comments, that's a win for all of us. Me included.
So to start with: American Empire. Although it's something of a slander on Burr's actual political beliefs (such as they can be teased out), the only really good shot at Empire early enough to get us going is Burr's victory over Jefferson in the 1800 election. Fired by Bonapartism, he brings the U.S. into the Napoleonic Wars. N. still sells us Louisiana (he's still short of cash), and President Burr puts his Western supporters in charge of the war effort -- Andrew Jackson conquers Canada. (Yes, this is a huge demographic and strategic stretch, likely beyond even Jackson's robust skills. But we can't have the British Empire hanging off our northern frontier if we're going to have a proper collapse 120 years later. We just have to special plead for some incompetent British leadership and Jackson being Jackson a few years early.)
Burr disregards the Washington precedent, serves as President For Life. Purges, loyalty trials, Committees of Public Safety, and the whole Bonapartist rag-bag comes with; Burr rewrites the Constitution repeatedly to strengthen the Executive, Bolivar-style. Jackson is his successor, establishing a clear precedent of General to President. He takes Florida and Cuba. We've got our big, militaristic Empire; the army has to stay big to grind out wars against Tories and Indians. Let's say President Polk, or rather Zachary Taylor, conquers Mexico, which we almost did in OTL. Now we've got way too big an empire to run from Washington City with 19th century c-and-c technology; territorial governors-general gain more power. Burr, Jackson, and Taylor keep things strong in the center for now. We really need another century or so for the empire to settle into decadence, but we don't have that.
It's tempting to time the first Time of Troubles to Taylor's sudden death in 1850 (just like in OTL). Maybe Winfield Scott's ascension ticks off the slave-owning generals; we get a civil war on the Roman model, The War Between the Generals. Scott wins, but only by making concessions to his supporting generals, McClellan and Butler and so forth. He dies in 1866; McClellan takes over but alienates virtually everyone ...
... including the British and French (and etc.) who have been biding their time. They sold arms to all the anti-Scott generals during the War, they enforced "free trade" with the ports those generals controlled. When McClellan takes over, he tries to muscle the Great Powers out of Charleston and New Orleans and Veracruz and Quebec City ... and discovers he can't do it, especially since his fellow generals (used to more latitude during the War and in no hurry to hand over power to the "Little Napoleon") don't willingly back his plays. Forty years of problems come next; minor trade wars, a European punitive expedition that burns Memphis or San Francisco; European companies get "leases" for American gold fields and oil wells and their loans build railroads to their chosen "free trade" ports. They start carving out their own spheres of influence: Britain starts trying to detach Atlantic Canada or New England; France covets Mexico and Louisiana; Germany starts building a new port in Florida; all the Powers jointly administer Lower Manhattan as an International Settlement in New York. American generals start "filibustering" against European interests; local governors and the President's Cabinet wink at such behavior. Some of them back Socialist strikers to shut down European-owned interests; this adds more unrest still, and the Socialists driven out of Germany and Russia can always find some American governor-general to give them entry papers, if only to undermine their neighbor in the maneuvering to replace the ailing, gouty President William R. Shafter.
Then World War One breaks out, and the Europeans have to leave America alone for a bit. William Jennings Bryan outmaneuvers his rivals to take hold of the massive Second Revolutionary Movement, which strikes when Shafter's successor Arthur MacArthur dies in 1912; nobody wants to swear to 32-year-old Douglas MacArthur (Arthur's obvious heir-designate) so General Nelson Miles declares for the Revolution, and we have a big, ramshackle Republic ready for the collapse when Bryan's complete unfitness for executive command begins to kick in. Especially if the Great Powers keep trying to drag America into WWI as a source of cannon fodder; maybe individual generals take mercenary contracts, maybe we have rebellions in Mexico under Villa and Zapata and in Quebec under Laurier; after 1917, a mad White Romanov general rides down from Alaska to try and found a Rasputinite colony in Utah based on free love and jimson weed. Without America, WWI drags on until 1920 or 1921 (as it did in Turkey and Russia-sort-of in OTL) and ends with everyone tired and exhausted and wanting to sell a whole lot of war surplus weapons to somewhere else or go broke.
Then Bryan dies in 1925 (same year as Miles) and Hell really lets out for breakfast.