Theory of everything differ both from gravity and from electromagnetism. A further hurdle was the acceptance that in a TOE, quantum mechanics had to be incorporated from the start, rather than emerging as a consequence of a deterministic unified theory, as Einstein had hoped. Gravity and electromagnetism could always peacefully coexist as entries in a list of classical forces, but for many years it seemed that gravity could not even be incorporated into the quantum framework, let alone unified with the other fundamental forces. For this reason, work on unification, for much of the twentieth century, focused on understanding the three "quantum" forces:
Unified combatant command unified combatant commands is legally mandated in Title 10, U.S. Code Sections 161–168. The Unified Command Plan (UCP) establishes the missions, command responsibilities, and geographic areas of responsibility of the unified combatant commands. As of May 2018, there are ten unified combatant commands. Six have regional responsibilities, and four have functional responsibilities. Each time the Unified Command Plan is updated, the organization of the combatant commands is reviewed for military efficiency and efficacy, as well as alignment with national policy. Each unified command is led by a combatant commander (CCDR), who is a four-star general or admiral. CCDRs exercise combatant