Once upon a time, there was a place called the Holy Land. This little area was special because several of the world's biggest religions saw it as their birthplace, and as hard it is to believe today, these religions liked to fight over it. Today, these fights have sweet, multi-word names with words like "Storm" and "Freedom" in them. Back then, people just called them "Crusades".
There were a whole bunch of Crusades during the Middle Ages, most of them between the two biggest religions of all: Christianity and Islam. For hundreds of years, Muslims had controlled the Holy Land around Jerusalem until the Christian armies of the 1st Crusade won it back, before promptly losing it again in the 2nd Crusade. There would be many other attempts to regain the Holy Land, and many more Crusades, but by far the most ridiculous of these was the 4th Crusade.
It was the year 1202, and there was this Pope named Innocent III. Despite his name, Innocent III was really hankering to kill some Muslims and win back the Holy Land, which the Christians had embarrassingly failed to do in the 3rd Crusade. His plan involved amassing a whole ass-load of knights, sailing them over to Egypt, and then watching them stomp their way up toward Jerusalem from the southwest while he watched safely from the comfort of his comfy pope chair and silly pope hat.
But there was a problem. To ship an ass-load of knights, you need an ass-load of ships, and the only city that had the capacity to cargo-ship a whole Crusade's worth of soldiers and supplies was Venice. And Venetians didn't do shit for free. Venetians DO, however, love making deals, so one was struck: the Crusade leaders would pay about 85,000 marks, and Venice would even throw in some more boats of its own, if they could share in the spoils. They were some enterprising mo-fo's, those Venetians.
There was another problem. After three Crusades, some people were tired of fighting, and many of the groups were disorganized and squabbling amongst themselves. Also, many of the potential Crusaders were French, and just wanted to just wanted to sail directly from Marseilles without going to Venice first, so they could sit around eating cheese and not bathing for as long as possible. So when the "massive" crusade army showed up in Venice, it was only about a third as massive as it was supposed to be, and only had about a third of the money.
But those clever Venetians had a solution.
"OK, so you can't pay," said Enrico Dandolo, one particularly sneaky Venetian. "But maybe we can arrange a little deferment... if you guys can do something for us."
"What do you want?" replied the Crusade leaders, already suspecting that this particular Crusade was not off to a wonderful start.
"Well, see, there's this little city down the coast, called Zara. They're been rebelling a little, back-talking us, and so on... Tell you what. You guys have a big army. We'll defer your payment until later if we can stop and sack Zara on the way."
"I don't think the Pope's going to like that very much. Zara's a Christian city."
"Well maybe 'the Pope' doesn't need to know about it," said Dandolo, touching his finger to the end of his nose.
What could the Crusade leaders do? The only alternative was to give up the Crusade, break their Crusading vows and return home broke and humiliated. And so off they sailed to sack Zara, which didn't take too long considering it was just one little city and it was being attacked by two-thirds of an entire Crusade.
The Pope, of course, did find out, and excommunicated the whole bunch. And so, 1,500 miles from the Holy Land, the 4th Crusade was doing great.
Meanwhile, 500 miles west in a little down called Constantinople (not Istanbul), the former ruler Isaac II Angelus had been deposed, blinded and Imprisoned by his brother, who ruled as Alexius III (because that kind of shit went on all the time back in the Middle ages). Isaac's son, who preferred his eyeballs intact, fled the city, and by miraculous coincidence (or perhaps masterminded treachery) ended up in Zara as the Crusaders were finishing slapping everybody around.
"Hey Crusaders!" Little Isaac probably yelled as he ran up to the army, because that's what I would have called them if I was alive back then. "If you guys go to Constantinople with me and help put me back on the throne, I'll take care of the rest of your debt, return the Eastern Orthodox Church there to the Pope's control, and you can do a little plundering too!" Well, Dandolo and the Venetians liked the idea of getting their money back (and the idea of spreading their greedy influence, which may have actually been the whole motivation behind the plan all along), the Crusaders liked the idea of spreading Christianity and funding further conquests, and everybody liked the idea of a little plundering, so off they all went to Constantinople, which was not at all on the way to the Holy Land.
After all, the Crusader figured "Why not? We're already excommunicated; might as well do some raping and plundering."
Well, Alexius III took one look at the invading Crusade fleet and got the hell out of dodge (though not before making off with a bunch of treasure), and the people of Constantinople let blind old Isaac Sr. out of prison and made him co-emperor along with Isaac Jr., who ruled as Alexius IV. But of course people didn't like Alexius IV, who first of all had been put into power by greasy Latin invaders and second of all taxed the shit out of them in order to afford gifts to give to all the nobles, to regain their favor. This also meant that Alexius didn't have enough money to repay the Crusaders, who waited around to get paid and pissed everybody off even more by running up a ginormous tab at the local bar. Oh, and they also burned down a local Mosque just for fun, which spread and subsequently burned down a quarter of the city.
Enter Alexius Murzuphlus, who led the anti-Latin group, called himself Alexius V, had Alexius IV killed (because that kind of shit went on all the time back in the Middle ages), put Isaac Sr. back in prison where he soon died, and fortified Constantinople's defenses against the Latins. By this time, the Crusaders were getting pretty sick of hanging out all the way up in Byzantium, the guy who had brought them here was dead, and they still hadn't conquered a single Muslim thing (except that Mosque that they burned down). So they decided it would be easiest of they just conquered Constantinople, installed a Latin as king, and used Constantinople's empire and it's resources to finally, finally, go to Egypt. And so they invaded again, though this time they were less polite about the sacking, because now it was just about conquest. Things actually got pretty out of hand, and the Crusaders sacked, plundered and burned the shit out of Constantinople - another Christian city - and it was never the same again.
And then, the 4th Crusade just sort of petered out. It had been two years since they started, the Crusaders were just as far from the Holy Land as they'd started, and the forces were smaller now. Some stayed in Byzantium to claim land, and everyone else went home to "recharge" for a bit before heading to the Holy Land, "a bit" turned into "a while." And so, a Crusade that was originally intended to conquer Muslim Egypt and reclaim the Holy Land ended up conquering two Christian cities nowhere near where Jesus was born, and helped forever break the link between the Eastern and Western churches.
But at least Pope Innocent - having temporarily won back religious control of Constantinople and thus forgiven everybody - was happy. And there was much rejoicing.