Allegiance (COD : MW 2019)

The Allegiance is a multinational military alliance featured in the multiplayer universe of Call of Duty: Modern Warfare 2019. 
They are mainly made up of the sizable remnant of Barkov's forces including the Spetsnaz detachment that refused to return to Russia to face charges of war crimes after being ordered to by their government. In order to combat both the west and Al-Qatala, they forged an alliance with the Pan-African militia called the Jackals and the Russian PMC Chimera. In response, NATO formed a military alliance consisting of the best special operations units at their disposal called The Coalition. 
Multiplayer Universe: The idea here is to definitively separate the Multiplayer version of the Allegiance from the Co-op version from Modern Warfare 2019. The developers have stated that the Multiplayer exists in a different timeline from the Co-op version of events following the Campaign. Whereas in the official Co-op story, after Barkov was disavowed following the reveal of his cruelty and crimes against Urzikstan, his Forces were disbanded by the proper Russian government and recalled back to Russia. Most complied but a remnant of them went on to join the Ultranationalists and fund Khaled al Asad's Al-Qatala to fight the west. In multiplayer, Barkov's forces refused to go back and founded the allegiance to keep oppressing Urzikstan and fight the the Coalition around the globe. I hope this helps everyone understand the difference between the two!