The reason Lazar should have taken the deal was because the Sultan had a huge army, at least 40,000 men, a massive number for pre-antibiotic days. And maybe 4,000 of those were the Janissaries, Christian boys grabbed from their mommies as a kind of infidel tax, taken to Istanbul to be brainwashed into Muslim fanatics and turned loose on the Sultan’s enemies. You have to admire that, taking the little infidel kiddies and turning them into Muzzie stormtroopers. I mean, just because you’re a world conqueror doesn’t mean you can’t have a sense of humor.
Lazar’s Serbs had a pretty good force of their own, maybe 20,000 men--including a few Croats, which is really amazing because if you know anything about the Balkans you know Croats go completely apeshit with hatred when you even mention Serbs, like that big jock in the movie who used to sniff the air and go, "NERDS!" when some math geek was in smell range.
But the Croats could see the Turks coming their way, and had the sense to fight with the Serbs to try to stop them before they reached Croat-land. It’s actually pretty classic gang-war logic: the 12th Street boys may love to fight the 14th Street kids, but if some gang from out of town shows up, they’re going to unite against it. Or pretend to. Because that’s the other classic element here, treachery: one of the Turks’ big assets was a traitor Serb noble, Dejanovic, who knew the territory and acted like their Indian scout, hoping to share the spoils.
Serbs having a hard time "getting over" the loss of Kosovo
The Serbs were fighting on their home field, but the Turks were professionals, vets with dozens of battles all over the Balkans to their credit. The Turks also had clear superiority in armor and weapons over the Serbs, who had panic-mustered every stable boy and dirt farmer they could find, even if they had no armor or proper weapons. One of the coolest features of the Serb force is that they had what European armies never seemed to have: mounted archers. Even so, most accounts of the battle spend a lot of time talking about the powerful volley of Turkish arrows that started the battle, so reading between the lines--which you absolutely have to do to make any sense of these old ballads--it seems like Kosovo started out as the classic encounter between European tactics, shield wall and heavy cavalry, vs. Steppe warfare: long-range arrow bombardment and maneuver.
The Serbs did what European armies always did best: they charged, and smashed right into the Ottoman force. Eastern armies were always impressed with what those white boys on their big plow horses could do on a flat field, with room to get up speed. There’s an Arab saying that dates from the Crusades: "The charge of a Frank (European) could knock down the walls of Babylon."
But there’s another truism about cavalry charges: unless they were supported by infantry, cavalry battles usually dissolved into "melees," meaning a bunch of individual duels between sweaty grunting tired guys on sweaty grunting tired horses. A few bold horsemen can make a big dent in the enemy line, but if the enemy has the discipline to stay in formation and the numbers to plug the dent, then eventually numbers will tell.
That’s what happened at Kosovo, as the day wore on and everybody’s hacking arms got tired: the Serb charge was absorbed, stopped and finally reversed as the Turks committed more troops to battle.
There are times in war when courage is a bad idea. After Stalingrad the Germans should probably have surrendered on the Western Front, applied for admission as the 51st State and hoped for the best. All they got for the long years of hopeless fighting after that defeat was a few million casualties and a badass rep that got their logo put on a lot of bikers’ helmets. Not much of a return on investment.