In the early-mid 1960s, inspired by the idea of the Holy Grail War, namely the Third sometime in the 30s or 40s, a multinational (read: both Soviets and otherwise) group of magi and scientists work to create a similar project. Realizing that, if humanity can split the atom then hey they can probably adapt that level of science to magecraft and create something really fancy. The goal of this is to settle the Cold War, reacting to increased tensions after the crises of the late 50's and early 60's such as the Cuban Missile Crisis. Named the Manhattan Project II as a way to hide it as yet another nuclear weapons project, the scientists and magi create their own replica of the Holy Grail, except, y'know, not tainted. The winner of the subsequent War, as a magi from either a NATO or Warsaw Pact country, would use their wish to end the Cold War with benefits to their home country. Of course, that's a definite oversight by the designers as the magi who participate learn the scope of the Manhattan Project and for the most part change their goals to more personal ones. Being untainted and the result of considerable magic and nuclear science, the Manhattan Project's replica Grail has an enhanced ability to contain mana and the power of Servants, as a design choice to enable members from every Warsaw Pact state and most NATO states to participate in the replica Holy Grail War. Due to the influence of the Holy Grail War on the Manhattan Project II, some things were kept as flavour, and others as design requirements: including the seven standard classes of the Grail War was necessary as to allocate mana more effectively by grouping the many Servants up, there are less Berserkers than other classes due to their high mana requirement, and only one Assassin, yet again Hassan, is summoned because of the fact that only one representation of any mythological or historical figure can be present, and Hassan-i Sabbah is the best fit for the Assassin archetype. Because the Throne of Heroes is entirely unconnected to the Manhattan Project, the Servants exist in-the-flesh: they are direct manifestations of the myths and stories of what would in the Grail Wars be Heroic Spirits, and so when they die they return to their places in history, unless they win the Manhattan Project. They are fundamentally magical beings when they are summoned, but ultimately, if you cut them they bleed. They're more like Saber or F/SN Gilgamesh in that way. The Servants are the actual people and at the same time not; they're amalgamations of the concept of the famous people based on who they actually were and the legends surrounding them. Other Heroic Spirit/magic rules apply, though: the older they are the more powerful they are (due to their concept having more history/experience) and the more well known they are, again, the more powerful they are, because their concept is stronger. There is no rule that states that there must be one Servant for each class, or that each class must have an equal number of Servants. The presence of Servants instead of some more scientific, non-magic deciding factor is based on the idea that secret magic is easier to conceal than all out fighting. Who believes in magic, anyways? Ha! The Manhattan Project selects heroes mostly at random, due to the collection of related artifacts being tedious, and the creators of the project wanting to get it over with already. For the most part the first summoned Servants for each class are those who fit that class best, aside from the Berserkers who, as per the Holy Grail War have a special summoning ritual, and the Servants of the magi who specifically choose who they summon. As a result some of the summoned heroes are considerably h4x due to being nearly the best in their class, such as Moses and Scathach for Caster and Achilles for Lancer. The setting is Washington, DC for its strong leylines as a result of the amount of history the place has in America, with the high number of monuments and because it's the capital of the USA. For symbolic and logical purposes the war is presided over by a member of the Church, hanging out in the Oak Hill Chapel.