At the time of the First Crusade in 1095 most of the Middle East was (well, theoretically) under the authority of the Abbasid caliph in Baghdad. But the caliph was basically a religious figurehead at this point - the crusaders noted that he was somewhat equivalent to the pope (although the pope was significantly more powerful).

North Africa and Egypt was originally part of the caliphate too. But in the 9th century Egypt (and southern Palestine/Syria) broke away under the Tulunid dynasty. In the 10th century Egypt was conquered by the Fatimids, who were a Shia dynasty (as opposed to the Sunni Abbasids) from further west in North Africa.

At the same time, Persia and areas further to the east in central Asia came under the control of Persian and Turkic dynasties. The Persian Buyids captured Baghdad in 945 and the Seljuks took it a century later in 1055. The Seljuks had been gradually converting to Islam for a few centuries already. There were also a lot of converts to Christianity and Judaism, as well as those who continued to practise their traditional shamanism. But the ones who converted to Islam became zealous supporters of the Sunni caliphate, even as they took over all of the caliph’s temporal powers. The Seljuk sultan in 1055 was Tughrul Bey, who was succeeded by his nephew Alp Arslan.

The Seljuk empire in Persia and Baghdad is generally known as “Great Seljuk” but many Seljuk princes and adventurers continued to move further west and south into Anatolia and Syria/Palestine. Alp Arslan defeated the Byzantine Empire at the Battle of Manzikert in 1071. Jerusalem was also captured around the same time by the Seljuk mercenary Atsiz.

Alp Arslan was assassinated in 1072 and was succeeded as the sultan of Great Seljuk by his son Malik Shah. But his empire didn't last long - in 1077, one of Malik Shah’s relatives, Suleyman ibn Qutalmish, established the independent Sultanate of Rum (literally “Rome”, since that’s what both they and the Byzantines called the Byzantine Empire). Meanwhile Malik Shah sent his brother Tutush to govern the territories conquered by Atsiz in Syria and Palestine. Tutush got rid of Atsiz (he had him imprisoned and eventually executed) and governed from Damascus, while he placed Jerusalem under another Seljuk warlord, Artuq.

Rum/Anatolia was apparently not sufficient for Suleyman though. He invaded Syria in 1086 but was killed in battle against Tutush at Aleppo. The governor of Aleppo at the time was yet another Seljuk warlord, Aq Sunqur. Tutush had Aq Sunqur executed a few years later in 1094.

In 1092 Malik Shah was assassinated. Suleyman's son Kilij Arslan, who had been imprisoned after Suleyman's death in 1086, escaped back to Anatolia and reasserted the independence of the Sultanate of Rum.

Malik Shah’s infant sons Mahmud and Berkyaruq were proclaimed sultans by different factions - Mahmud was assassinated too but Berkyaruq managed to rule for another ten years. Tutush took advantage of this situation to declare himself the independent Sultan of Damascus. He tried to take over Great Seljuk as well, but he was killed in battle against Berkyaruq in 1095.

Tutush was succeeded by his son Duqaq. His other son Radwan took control of Aleppo. Palestine was governed by Suqman and Ilghazi, the sons of Artuq (who had died in 1091). Further north, in 1096 Mosul in Mesopotamia fell under the control of another Seljuk soldier, Kerbogha - he had been one of Berkyaruq’s supporters but was captured and imprisoned by Tutush. Radwan released him after Tutush’s death.

Whew...everything clear as mud so far? Hopefully I haven’t lost you yet! I know this is a pretty bewildering amount of names and places.

By the time the crusade was getting underway in Europe in 1096, this is what was happening in the places they would eventually conquer. It was just a coincidence that the crusade happened to arrive at this very chaotic moment. The crusaders definitely benefited from the lack of unity and cooperation between all these Seljuk rulers, who often preferred to fight amongst themselves rather than against the invading crusaders.

The crusaders fought against Kilij Arslan in Anatolia but successfully made their way to Antioch. Both Duqaq and Radwan arrived to relieve the crusaders' siege, but both were defeated. The crusaders took Antioch with help from an Armenian guard who secretly let them in through one of the gates, but they were immediately besieged themselves, by Kerbogha of Mosul. They unexpectedly defeated Kerbogha and continued south to Jerusalem.

But by now, in 1098, the governors of Jerusalem, Suqman and Ilghazi, were expelled from Palestine by the Fatimids of Egypt. So it was the Fatimids, not the Seljuks, who had to face the crusader army when it arrived the next year in 1099. The crusaders of course captured Jerusalem in July 1099, and defeated another Fatimid army a month later.

The Fatimids continued to attack the crusaders in the years after the crusades, and so did the Seljuks. Duqaq and Radwan were still around for many years. Suqman and Ilghazi carved out their own territories up in Mesopotamia; Ilghazi even ended up as far north as Georgia, but the Georgians defeated him at the Battle of Didgori in 1121. So the Kingdom of Georgia emerged as a major power in the region, sort of indirectly as a result of the crusade.

A more direct result of the crusade was the emergence of an independent Armenia in southwestern Anatolia. Previously the Armenians had been ruled by the Byzantines or Seljuks, but now that the crusaders were in control of Antioch and Edessa, the Armenians took the opportunity to establish their own state as well.

Meanwhile, Aq Sunqur’s son Zengi ended up recovering Aleppo, and he took Mosul too in the 1120s. Zengi destroyed crusader Edessa in 1144, which led to the Second Crusade in 1148. The Second Crusade and tried (and failed) to conquer Damascus, which then united with Aleppo and Mosul under Zengi's son Nur ad-Din. One of Nur ad-Din’s generals, Saladin (who was actually a Kurd, in the service of the Seljuks), took control of Egypt and overthrew the Fatimid caliphate in 1171. Saladin also took Jerusalem back from the crusaders in 1187.

So, if I can summarize very briefly, the Seljuks of Central Asia took over the Abbasid caliphate and established the Great Seljuk Sultanate in the 1050s. By the 1070s they had also spread into Syria and Anatolia, but Great Seljuk mostly fell apart and independent sultanates were established in its place. The Syrian territories fragmented even further so that by the time the crusade happened to arrive in 1097, there was no longer one united state to resist it. But the presence of the crusader states in the east inadvertently led to the union of all of Syria and Egypt.

Sources:

My main sources for this are:

P. M. Holt, The Age of the Crusades: The Near East from the Eleventh Century to 1517 (Longman, 1986)

Thomas Asbridge, The First Crusade: A New History (Oxford University Press, 2004)

But for more information on the crusades from the Middle Eastern perspective, there are a lot of good places to start, especially:

Carole Hillenbrand, The Crusades: Islamic Perspectives (Routledge, 1999)

Angeliki E. Laiou, and Roy Parviz Mottahedeh, eds., The Crusades from the Perspective of Byzantium and the Muslim World (Dumbarton Oaks, 2001)

Paul M. Cobb, The Race for Paradise: an Islamic History of the Crusades (Oxford University Press, 2014)

Niall Christie, Muslims and Crusaders: Christianity's Wars in the Middle East, 1095-1382, Routledge, 2014.