"Knights, why do you still think? You who are prized for your armed deeds, offer yourself to the one! who was raised upon the cross for you, and yours, sake!" - Flourished line from the Chevalier, Mult Estes Guariz.
1239-1242 - The Sultanate of Egypt had been in shambles for over a decade following the death of Al-Adil, brother of Saladin himself. This shambled setting was put in order after much effort by Al-Kamil, the son of Al-Adil, with the Sultan continuing what his father had started and putting further pressure on the Crusader States in the Levant [1]. This had ensured a further decade of 'peace' to rebuild, but left the Muslims unprepared for the effectively sudden arrival of a new Crusade [2].
At first it was nought but minor skirmishes, as early as February, along the borderlands--mostly set in motion by the movement of K.o.J based troops. These would only escalate as the actual Crusade itself began to shake-off. The planned functionality was simple; the army would be split up--allowing for the two Emperors to uphold their duties in the Middle East, while the eager King Louis IX would take a 3rd of the Crusading force to Egypt to launch a strike at the core of the Sultans capacity.
The Crusaders under Louis would arrive at, and occupy, the port of Damietta in late March, before making the hard march south towards Cairo in early April (after securing their front). Between them and Cairo lay Al-Mansoura; which blocked their northern route from the north side. This forced the Crusaders to lay siege to the city; however Louis' scouts scouts failed him, and did not warn him and his forces of a large Muslim army led by Al-Kamil himself.
The Crusaders were running out of supplies, and were forced to turn back lest they be forcibly battered away between the walls of Al-Mansoura and the Sultan's armies. Louis set up camp in a good position, just north, attempting to draw in the Muslims--yet his scouts would fail him once more [3]; not warning him of the fact that Al-Kamil had made his next move.
The Sultan ordered the destruction of the dam managing the nearby section of the Nile River; flooding and effectively destroying the hard-built camp before the Crusaders even had time to react. This would be compounded by a shattering blow against the Christians the following morning as they tried to back out to more favourable ground; with the army of Al-Kamil bloodying the Crusaders heavily; and forcing them up the nearby hill.
It would be a bloodbath; one that saw most of the Crusaders killed--yet the skill at arms, and leadership, of Louis IX allowed them survive and break out; carving a path in a slow, yet functional manner, back to Damietta, fleeing Egypt back to Acre on whatever ships were left. When Louis and his forces returned to friendly territory what had been an army of hardened Crusaders numbering 13,000 numbered now 4,000; leaving the total force at around 31,000.
Louis himself took the notion on the chin; willing to accept full responsibility for the failure; yet the cries of his enamored soldiers that it was not his fault rang loudly--and saw John and Frederick note that Louis had done the best he could. This whole event had wasted 3 months, leaving the Crusaders in their totality in late July with only one option; collective campaigning.
What would follow, over the next 2.5 years was nothing short of a dustbowl--the forces of Islam hardening at every blow offered them as the three-prong Crusader force; aided by those of the K.o.J, and a reluctant Antioch. The largest non-siege battle of these years was the Battle of Acre; as the aging Al-Kamil pushed against the Crusaders, and their current backers in the Crusader States, with a direct stab at the heart of what was left.
The battle began in late September of 1241, and was dragged on throughout a period of 4 days as forces joined and retracted intermittently. It was only the skill of Louis himself--dedicated to righting the wrong he felt responsible for--that truly held the line; as the German forces under Frederick proved too integral to the center of the Crusader force to risk continued battle-movement, and the Romans relied on the signals given by Louis [4]. It would end in a decisive Crusader victory--with Al-Kamil maimed in close-combat with a Lakonoi, forcing his core forces to flee. The battle had cost the Romans dearly though in the death of the aged Andronikos Romanos--the first Captain of the Lakonoi, placed by Romanos himself. Andronikos would be replaced by his chosen successor; Romanos Turkas.
It is said, within several contemporary sources, such as the scholar Henry of Ravenna, that at the end of the battle--when the major prisoners were being rounded up--that an Egyptian officer had the bravery to demand why the Turks, after looking at the Lakonoi, were fighting alongside the Christians--let alone the Romans. In response, it is said, that Romanos Turkas asked in fluent Greek; "Ti Toúrkoi?", "What Turks?" [5], before carving out the eyes, and upper face, of this officer in one blow of his cleaver.
Al-Kamil would die broken in body at the end of September; leaving his unready son as the new Sultan; Al-Adil II. Al-Adil II was a military man, but not a ruler--this was shown in his blunt, and blatant, disregard for protocol and functionality upon his ascension. The young Sultan simply wanted this whole mess with the Christians, as he saw it, over with. His skilled response in leading counter-pushes against any attempts to carve out further territory in the Holy Land by the remaining Crusaders over the rest of year; not even breaking to allow them Christmas, put the fear of failure in those warriors of God.
John was the one to suggest the riskiest move of their efforts; retaking Jerusalem itself. It would send a message to the Muslims that it was over; and to cease hostilities, while giving hope to those Christians who fought against the Muslims. It would be in early January 1242; forgoing the usual month-long preparation done each new year, that the united forces of the Christens, by now numbering around 29,000 due to gradual losses, would make for Jerusalem.
The siege was dragged out; made harder than ever expected by Al-Adil II's skilled hit-and-run raids into sections of the brokered siege camps. In the end the Crusaders, after two months on this, were getting desperate; stripping much of their siege equipment to allow the Roman engineers to build functional counterweight trebuchets to cause more damage to the hardened walls of the Holy City.
Funnily enough, this was not even needed as time panned out. As-Salih, the exiled half-brother of Al-Adil II, had returned--and led a skilled campaign against his half-sibling in retaliation for the loss of his rights. This movement caused an anticlimactic end to the Great Crusade; as Al-Adil simply offered Jerusalem and its environs as payment to the Christians so that they would, in short, sod off and let him handle his own internal affairs. This would be finalized in April 1242.
While Christendom had the city back the price was heavy; only 19,000 Crusaders remained.
In the end, Frederick II's forces had taken the major losses--even after Louis' own early losses, as the Germans had acted as the core of the Crusader force, with the Holy Roman Emperor returning home with little more than minor prestige to show for it (aside from the obvious loot and so on).
Louis IX himself came off a hero, as the French troops he led spoke of his bravery, skill and piety more than was expected--and often said notes of his friendship with the 'Great Emperors' simply gave him more political clout and prestige upon his return to France.
John III? Life as a campaign leader in the Near East had taken its toll on the now 41 year old Emperor--who began to get sicker and sicker as the time dragged on; unable to be moved from Acre upon his arrival until November. He would return with what remained of the Lakonoi; a force of 4,500 or so, a sickly and bedridden Emperor who regularly coughed up blood.
In his moments of pain-induced delusions he was noted to cry; "Here, Trajan![6] He lines out my path!"
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[1] While the Crusader States had managed to maintain a firm grasp on the Middle-Eastern Coastline, they lacked the staying power to push in-land. This was proven when they were effectively cracked over the head by Al-Adil during their attempt; leaving a unique stalemate in the air as the aged Al-Adil was forced to effectively call a time-out to deal with successional issues.
[2] It hadn't quite sunk in yet that the Romans, and their Latin fellows in Christendom, had become firmer allies since last time--as in the 3rd Crusade the Roman leadership effectively tried at every turn to hamstring their Latin fellows.
[3] Unlike the Romans scouts, who were masters of the terrain, the scouts employed by Louis IX could never get used to the ever shifting climates, rivers and basic territories of Egypt.
[4] Due to John's sickly nature while on campaign he could do little more than command in a functional manner--and often relied on others movement notions, namely Louis, to angle his forces correctly. This was offset by the fact that the Lakonoi were the most elite formation there--and were relied upon for much of the hard hitting victories within the area.
[5] The Lakonoi fully considered themselves Roman after their upbringing as elite soldiers for the Emperor and his Empire. It was an insult to call them, as they stood, Turkish--even if several notions such as their marching functions, and tactics, had a Turkish flare.
[6] Trajan's death was due to natural causes, but its onset was due to the harsh lifestyle he was forced to endure while campaigning in the Near East, and beyond. This cry of John's was likely his well-read mind playing the notes, in tandem, for him and all those who could hear him.