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If it’s fair to say that the Fourth Crusade’s sacking of Constantinople discredited the whole enterprise, and in my opinion it is, the Sixth Crusade wrung the last vestiges of seriousness out of the movement. Ironically, of the numbered Crusades it was one of the more successful—certainly more successful than the disastrous Second, Fifth, Seventh, and Eighth Crusades and the...whatever the Fourth Crusade was. An argument could be made that it was the most successful numbered Crusade other than the First, since like the First Crusade it ended with a Christian (well, sort of Christian) ruler having gained control of Jerusalem. That's how the early Crusaders would've defined “success” and by that standard it’s difficult to argue that the Sixth Crusade was a failure, at least on paper.
I say “ironically,” though, because the outcome of the Sixth Crusade has to be tempered by an amazing number of caveats. For one thing, its leader had been excommunicated (hence my “sort of Christian” remark above), which gives this Crusade the unique distinction of having been actively opposed by the Church. For another thing, it involved barely any actual fighting. It was, instead, a long negotiation between that excommunicated Crusader king—who didn't really want to be on Crusade, didn’t really have much of an army with him, and had no support from the Church—and the leaders of an Ayyubid sultanate that was by this point so decrepit that it didn’t dare risk going to war even with a guy dealing with all of those other problems.
The Sixth Crusade was, if you want to be crude about it (and why not?), Holy Roman Emperor Frederick II’s (d. 1250) middle finger to Pope Gregory IX (d. 1241). Frederick was undeniably the most powerful European monarch of the period, but he also had this habit of talking about going on Crusade without actually ever going on Crusade. Frederick had promised to take up the cause at his coronation as King of Germany in 1215, and dramatically renewed his pledge when he was crowned emperor in 1220, but he nevertheless waffled on joining the 1213-1221 Fifth Crusade, which attacked the Ayyubids in Egypt. Throughout that campaign’s siege of Damietta and the army’s deleterious year of inactivity after finally capturing the city, Frederick kept sending messengers to the Crusaders promising that he was on his way. Some German forces did eventually arrive at Damietta, just in time to join the army’s disastrous march south, but Frederick himself, and the vast army he supposedly led, never arrived.
As you might expect, some people took a dim view of Frederick after the Fifth Crusade went bust. Among them was Pope Honorius III (d. 1227), who rebuked Frederick for his inaction by letter in 1221. Relations between Frederick and the Vatican weren’t great anyway, but that was the norm between popes and Holy Roman Emperors, who were constantly feuding over issues of primacy in the Church. Frederick’s abandonment of the Crusade added a new and powerful grievance to what was already a strained relationship.
Frederick assured that pope that he had wanted to go on Crusade but just hadn't been able to leave Germany, and in his defense he really had been occupied with consolidating his rule over the empire. He promised to lead a great new Crusade that would leave in 1225, ten years after he’d first promised to take up the cross. Then in 1224, he told Honorius that he was going to need more time to put an army together. To fend off a possible excommunication, Frederick assured Honorius that his Crusade would leave in August 1227, and even signed a document that made it clear he would accept excommunication if he failed to leave at that time.
In the meantime, Frederick had been widowed, and his search for a new bride fell upon Isabella, the daughter of the titular King of Jerusalem, John of Brienne. Since Jerusalem was back in Muslim hands by this point, the “King of Jerusalem” actually ruled from the city of Acre, but the title was more important than the city. John was reluctant to consent to the union, fearing that the much more powerful Frederick would then claim the throne of Jerusalem by marriage. Honorius, on the other hand, loved this idea because he assumed it would commit Frederick to going on Crusade, finally. Frederick denied that he had any interest in claiming the throne, and so John consented, and Frederick and Isabella were married in 1225. Frederick then, and you knew this was coming, claimed the throne of Jerusalem. And there was really nothing John could do about it.
So by 1227 Frederick was well motivated, both by his pledge and by a desire to legitimize his new kingship, to go on Crusade. It was also a very good time to attempt a new campaign. The Ayyubids were in the midst of a civil war between the sons of the previous sultan, al-Adil (d. 1218), and Frederick was in talks with one of those sons, al-Kamil, about trading Jerusalem for military aid. And so with the proverbial wind at its proverbial sails (and presumably some actual wind at his actual sails), Frederick’s army set out in August 1227 as promised. It's just that Frederick, uh, wasn’t with it. He’d been stricken by plague and headed to Naples to recuperate, and despite his promises that he was definitely going to head east just as soon as he could to meet up with his army and do that whole Crusade thing, don’t you worry, he seemed pretty comfortable staying there (in Frederick’s defense, he had legitimately been quite ill). The new pope, Gregory IX, was no longer interested in Frederick’s excuses and/or promises, and excommunicated him in late September.
Frederick finally sailed for Acre in May 1228. The Crusade, now led by an excommunicate, was—in technical religious terms—a gigantic mess. Gregory actually urged Frederick not to go (talk about doing a 180) and then sent letters to Acre warning everyone there that Frederick was not to be obeyed as he was not a legitimate Crusader (nor, at that point, a legitimate Christian). He even excommunicated Frederick a second time, I guess in case the first one hadn’t taken or something. Isabella even died shortly before Frederick left, which technically meant he no longer had any claim to be King of Jerusalem. But he went anyway. I've seen Frederick called an “atheist,” and while that’s meant to be derogatory and I don’t actually think he was an atheist (the idea seems a bit anachronistic, frankly) it is pretty clear that he wasn’t a big believer in papal authority.
The one development that did affect Frederick’s plans was the death of al-Kamil’s main rival for the throne, his brother al-Muʾazzam. Consequently, the Ayyubids no longer had any incentive to trade Jerusalem for Frederick’s military aid. After arriving in Acre (after yet another lengthy delay in Cyprus hashing out Jerusalem’s political situation) and spending some time wrangling with al-Kamil’s negotiators, Frederick marched his army to Jaffa to rebuild that city's fortifications, which was intended as a threat to al-Kamil. Frederick’s army wasn’t nearly the size it had been when it set out—it seems that when they arrived at Acre without their ruler, most of his men figured that he’d backed out of yet another promise to Crusade and decided to go back home. But this march to Jaffa was impressive enough, apparently, to convince al-Kamil to make a deal.
The deal, which was signed on February 18, 1229, let Frederick claim a technical victory and get the hell out of Dodge, but its terms were laughable. Al-Kamil gave Jerusalem to Frederick, but the city had to be kept unfortified, all Muslims living there had to be allowed to remain, and the city’s Muslim holy places would still remain under Muslim control. In other words, Frederick was given possession of the city in name only. When he heard the terms of the deal, the Patriarch of Jerusalem refused to formally crown Frederick King of Jerusalem in the Church of the Holy Sepulcher, and Frederick’s remaining army, largely made up of Knights Templar and Knights Hospitaller, rejected the arrangement as a paper victory.
Frederick made a brief entry into Jerusalem, where he may have crowned himself king (it’s not entirely clear), before returning to Acre, where the patriarch was actually raising his own army to march to Jerusalem and garrison the city properly. This, of course, would have completely wrecked the deal Frederick had cut with al-Kamil, and he therefore put a stop to it. Meanwhile, word reached him that Gregory and John of Brienne were campaigning against his possessions in Italy, so he had to leave. As a final gift to the Crusaders he was leaving behind, Frederick made sure to destroy all their siege engines, so they wouldn't get any funny ideas about marching on Jerusalem once he’d left. As Frederick rode to the harbor to board his ship home, we’re told that angry townspeople threw garbage at him.
Frederick recovered his Italian holdings, made peace with John and Gregory, and was restored to the Church in the Treaty of Ceprano in 1230. But Gregory excommunicated him again in 1239, when Frederick attacked and defeated the Vatican-allied Lombard League in northern Italy. He spent most of the rest of his life in a literal state of war with the papacy and was actually excommunicated again by Pope Innocent IV in 1245. Jerusalem, meanwhile, came back into Muslim hands in 1244, following a siege by a Khwarazmian (Central Asian) army that had been recruited for the task by al-Kamil.