In Metal Gear Solid V: Ground Zeroes, starting a mission will lead to a brief bit of opening narratives that covers at least one important element in the mission. This was dropped in Metal Gear Solid V: The Phantom Pain with the inclusion of briefing tapes for the missions replacing them.
"In an era when the world was still divided into East and West... The Boss, the Mother of Special Forces and hero of World War II, defected to the Soviet Union. Under orders from the U.S. government, her final apprentice, Naked Snake, took her life. But for The Boss, her defection was no more than a cover. She was nade to sacrifice herself to prevent nuclear war. That was her final mission. The Boss used Snake to ensure that the Philosophers' Legacy, the vast military fund under Soviet Army control, would reach America. For these outstanding "accomplishments,"Snake was awarded the title of "Big Boss." After turning his back on America, Snake began recruiting soldiers with the help of Kazuhira Miller, a man he first met in a war zone. In 1974, the pair constructed a secret base on the Caribbean. Snake's private army continued to grow, until their need for a deterrent prompted them to obtain a nuclear weapon. Meanwhile, Major Zero, the man who once worked with Snake to create special forces unit FOX, used the Philosophers' Legacy to expand his intelligence organization, Cipher. Cipher proceeded to spread its roots into the furthest depths of the United States. With no military of his own, Zero longed to rejoin his old comrade Snake, and sent his operative Paz to make contact. Paz attempted to steal the nuclear-equipped walking battle tank Metal Gear ZEKE, but was thwarted by Snake. Flung from ZEKE's cockpit, Paz vanished beneath the waves of the Caribbean.
"The ranks of Mother Base are made up of those recruited out in the field, primarily by Snake, and those who chose to join separately. In addition to mercenaries and former government soldiers, it is also home to researchers and other civilian personnel. Together, they form a group without borders, ideologies or isms, with no regard to age, gender, nationality or race."
"Snake's army has spies operating around the world. The members of this Intel Team investigate conflict regions, strongholds, and other places of interest. Their investigations cover force strength, logistics, supply routes, planned operations, and behavioral patterns at the site- even down to the names and faces of family members. This intel is then recorded to help devise infiltration missions. Contract work is also taken on as the situation warrants. Agents infiltrate target sites themselves, though choppers are available if emergency evac. becomes necessary."
"Following its victory in the Spanish-American War, the United States was granted a lease of a piece of Cuban soil which it placed under military jurisdiction. Even in the wake of the Cuban Revolution in the 1950s, America gave no indication it would vacate the site. Tensions took shape in the carpet of minefields laid by each side between the site and the Cuban mainland. The United States insists the base is subject to neither American nor Cuban law, and U.S. Marines currently guard a prison camp located at its southern tip."
"The Joint Chiefs of Staff (JCS), comprised of the service chiefs of the Army, Navy, Air Force, and Marines, is the U.S. military's highest authority. It reports to the Secretary of Defense and the Pentagon. A heliborne assault utilizes helicopters to drop infantry on a location. It is considered particularly effective for suppressing insurgencies as well as for conducting raids."
"The nuclear weapons disposal facility on Shadow Moses Island in Alaska's Fox Archipelago is attacked and captured by Next Generation Special Forces led by members of FOXHOUND. The terrorists have secured hundreds of nuclear warheads, and they're demanding that the government turn over the remains of Big Boss. Solid Snake is once again called back into duty in a top secret mission to deal with the greatest terrorist threat the world has ever seen. Once again, Snake heads into battle... alone." ―from "METAL GEAR SOLID" (1998)
"Our galaxy consists of some 300 billion stars. Around half are orbited by planets, and it is said that on average, conditions on two of a star's planet are suitable for life. No great leap of imagination is needed to believe the universe must be home to a myriad of life forms. But what sorts of intelligence would develop on these worlds? That is truly beyond our imagination. What's more, we cannot say that our universe is the only universe. At the quantum level, until a wavefunction collapses upon observation, Schrodinger's cat is alive and dead as a superposition of states. The moment observation occurs, these two possiblities branch into separate worlds, with the universe continuing to split infinitely. If the universe splits without end, it's possible it could eventually recreate scenes from the past. And if those things played out differently, it wouldn't negate what happened in our reality - a paradox would simply mean another future. The following is one possible story in one possible universe, born of paradox, after paradox, after paradox..."