Sith External Expeditionary Force

The Sith External Expeditionary Force, was a Sith/Imperial Expeditionary Force that was sent to aid the Republic during the battle of Umbara. When the Republic invaded the Battle of Umbara the Sith Empire was conflicted on how to handle the crisis, Senator Padmé Amidala and Jedi Grand Master Yoda traveled to Exegol to meet with both the Dark Council and later arrived on Dromad Kas to speak to the Imperial External Senate. The Sith eventually decided to send an expeditionary force to aid the Republic after much debating and arguing which of course was ended with the leader of the Sith External Military, Darth Nox and Admiral Galon Krysto stated that they must crush the Separatist on Umbara, whereas Darth Imperius pointed out that Umbaran system hosted a vital trade route.

By sending an Expeditionary Force rather than a full military force, the Sith Empire could claim innocence and protect their trade route while the Republic would have aid of the more powerful Sith Fleet and army.

Senate and Council Crisis
The Republic invasion of Umbara though in some way or form justified presented an issue for the Sith Empire, that being in the Unknown Regions trade routes were rare, with only the Sith and Chiss being able to navigate the dangers of the Unknown Region. Currently the Umbaran system was the site of a major Trade Route between the Sith Empire, and the New Mandalorians. This created a crisis within the Imperial Eternal Senate and the Dark Council, who while allied slightly with the Republic, they didn't want to lose a vital trade route.

After speaking with a envoy team sent by the Jedi and Republic. The Sith decided to send an expeditionary force to Umbara in an effort to defeat the Separatist and secure their trade routes.

Battle of Umbara
The Sith External Expeditionary Force arrived with a fleet of 3 Xyston-class Star Destroyers, and two Sith Cruisers and one sith battlecruiser to the Republic Navy, while the Sith ground forces landed and aided the Republic by targeting the Umbaran fuel depots and supply centers. Unlike the Republic the Sith had more success against the Umbarans, as Sith and Stormtroopers had more experience dealing with local forces and counter-insurgency tactics, this combined with the more ruthless tactics deployed by the Sith such as mass bombing civilian population centers, and destroying infrastructure. Though the terrain and bobby traps left by the Umbarans caused several causalities.

Betray of General Pong
When Jedi General Pong Krell betrayed the Republic, the 501st and 212th were assisted by the 204th Legion of Sithtroopers and Ma'lik Company, which was made up mostly of Jet Sithtroopers. The Sithtroopers led by their commander, particularly hated Krell as he bossed them around despite them being volunteer forces. When the 501st and 212th teamed up to take down Krell the 204th aided them in their efforts, which lead to an AT-ST being destroyed by Krell.