Ultimatum is an acronym standing for Underground Liberated Totally Integrated Mobile Army To Unite Mankind.
It is an anarchistic organization dedicated to abolishing the concept of nationalism in order to bring about world unification through the use of terrorist activity against governments, institutions, and other symbols of nationalism, while supplying arms to various subversives.
Ultimatum was created by Mark Gruenwald and Paul Neary in 1986 and first appeared in Captain America # 321.
Amazing Spider-Man/New Venom
The ULTIMATUM organization recently surfaced in the pages of ASM 654.1. The Countess Bianca Demonico was tagged by the new Venom at a party and implanted with a tracking device. Tracking her to ULTIMATUM's base in Uzbekistan he realizes they've kidnapped Yusef Kassim, banker to one of the Middle East's largest oil companies. After a skirmish with ULTIMATUM agents and their leader Flag Smasher Flash successfully rescues Kassim.
Marvel Cinematic Universe
Karli Morgenthau, a gender-swapped version of Karl Morgenthau, leads a terrorist group compose of super-soldiers who believe the world was better during The Blip. Instead of using Flag-Smasher as a personal moniker, the whole team is called the Flag-Smashers.