Unified field theory classical field theory and, two years later, that of Theodor Kaluza, who extended General Relativity to five dimensions. Continuing in this latter direction, Oscar Klein proposed in 1926 that the fourth spatial dimension be curled up into a small, unobserved circle. In Kaluza–Klein theory, the gravitational curvature of the extra spatial direction behaves as an additional force similar to electromagnetism. These and other models of electromagnetism and gravity were pursued by Albert Einstein in his attempts at a classical unified field theory. By 1930 Einstein had already considered the Einstein–Maxwell–Dirac System [Dongen]. This system is (heuristically) the super-classical [Varadarajan] limit
Unified combatant command the terms of Article II of the United States Constitution. Thereafter, the military CINCs would be known as "combatant commanders", as heads of the unified combatant commands. A sixth geographical unified command, United States Africa Command (USAFRICOM), was approved and established in 2007 for Africa. It operated under U.S. European Command as a sub-unified command during its first year, and transitioned to independent Unified Command Status in October 2008. In 2009, it focused on synchronizing hundreds of activities inherited from three regional commands that previously coordinated U.S. military relations in Africa. President Donald Trump announced on 18 August 2017 that