The Trope Namer was partially inspired by Poland riding to the aid of Austria during the Second Siege of Vienna in 1683. King Jan Sobieski of Poland is considered to have stopped the Ottoman expansion into Europe. It has the classic elements of calling an entire army (and king) against a sieging army and succeeding.
The international volunteers that came to help Republican Spain against the fascists.
Similarly during the Winter War thousands of Danes, Swedes and Estonians came to Finland's aid, along with other volunteers, most notable are the Hungarians, who had to skirt around WW2 to get to Finland. France and Britain also had plans to intervene against the Soviet Union, which might have had interesting effects on the course of the major conflict.
During Israel's war of independence, volunteers from all over the world (many of them WW 2 veterans) clamored to aid the Israeli side. Two of the American volunteers were later fined and one jailed.
Similarly thousands of people from all over Europe went to Greece to join up with the Greek bid for independence from the Ottoman Empire, including famed crazy Lord Byron.
Foreign volunteers for the Boer side in South Africa's Anglo-Boer War.
Occurred repeatedly during the Crusades, which were triggered by Constantinople's requests to The Pope for aid against the Turks. But as Crusade succeeded Crusade, relations between the Westerners and their Byzantine hosts broke down until 1204, when the Venetians bankrolling the operation convinced the Crusaders to turn against Constantinople and sack it bare. Though five more Crusades were to come, any sort of understanding between East and West was gone; when in their final hour the Byzantines swallowed their pride and asked for help again, only a few hundred Europeans bothered to answer.
The declarations of war on Germany in 1939 by members of the British Commonwealth can be seen as this. In World War One, Australians, New Zealanders and Canadians went to war because, as part of the British Empire, they were obligated to. In 1939, it's because they chose to come to Great Britain's aid, as did many Americans who crossed the border and became part of the Canadian military before 1941, and Irish citizens who put aside the country's neutrality to serve with the British (in some cases, entire units of the Irish Army resigned en masse, crossed over to Britain, and were quickly snapped up by a grateful British Army). Some Irish also joined the Nazis for a chance at vengeance on Britain.
Actually incorrect, while the Waffen-SS did turn into Nazi Germany's Foreign Legion with time, both the Indian Legion (recruited 1941 by the Wehrmacht, transferred to the Waffen-SS in 1944) and the Ostlegionen or Osttruppen were also Wehrmacht.
After fighting a bitter war of independence against the British Empire, the Union of South Africa remained an independent dominion of the Empire. As such, they were included in the British declarations of war leading up to both world wars, but if they had chosen to stay neutral there was really nothing anyone could have done about it. Many South Africans were against fighting for the British cause, but after debate and political upheaval, they did fight in both wars and Jan Smuts, hero of the Boer War, became a Field Marshal in the British army and a trusted adviser to Churchill.
This trope is inevitable whenever a conflict involves the Islamic world. Let's just say that a lot of Muslims from across the world are helping Hamas, Hizbullah, the Taliban, and the MILF. And their enemies wonder why they can't win against them.
On a different note, when the Soviet Union invaded Afghanistan it was considered a noble thing to join the Mujahideen resistance. This was encouraged by the USA and many Muslim nations. Among many, Saudi Arabian Osama bin Laden joined the resistance. He later went beyond this when he founded al-Qaeda to go beyond assisting the resistance to creating a Wahhabi Caliphate across the region.
The Battle of Chalons in 451 CE. It's considered one of the first battles fought, not between two or three entities, but rather two coalitions. The Romans had the Visigoths, Franks, Armoricans, Saxons, Alani, Burgundians, and Sarmatians. The Huns had the Ostrogoths, Gepids, Rugians, Scirii, Thuringians, Scythians, Bastarnae, Taifals, and Alamanni. Both armies were of roughly equal size. The Roman coalition was led by Flavius Aetius, possibly the last great Western Roman general, and the Huns were led by Attila. Most historians, since the time of Gibbon, consider the battle to be epochal, with Western civilization at stake.
During the Soviet-Afghan War, people from all over the Muslim world went to Afghanistan to fight against the invaders.
The same thing happened again during the US-Iraq War. However that happened after the invasion. And the "helpers" managed to kill a lot more Iraqis than Americans.
The Balkans. The three major factions are Croat, Bosnian and Serb. Croats were traditionally German aligned and Catholic; Serbs Russian aligned and Orthodox, Bosnians were Turkish aligned and Muslim. So each side had volunteers and arms given, from Muhajadeen to volunteer adventurers, idologues, Neo-Nazis, let alone French, Russian and American arms to various factions. Then NATO invaded...
Chile, which began as a Spanish colony, ousted the Spanish authorities in September 18, 1810, and established a local government. Similar events took place at other cities in the Spanish America, such as Buenos Aires. This began a war between the new governments and those loyal to the Spanish monarchy (patriots and royalists, similar to the patriots and loyalists of the American Revolution). Chile was finally reconquered after the royalist victory at the Disaster of Rancagua, Carrera, O'Higgins and other patriots escaped to Mendoza (Argentina), at the other side of the Andes. There, O'Higgins worked with the Argentine José de San Martín to raise the Army of the Andes, that crossed the mountain range, counter-attacked the royalists and drove them away from Chile forever. Even more, O'Higgins could not arrive in time to the battle of Maipu, the final battle, which was directed solely by San Martín.
The evacuation of the Danish Jews during the Second World War: the non-Jewish people of Nazi-occupied (and, at least officially, collaborationist) Denmark disregarded their country's official stance not to mention their own safety to undertake a massive effort to hide their Jewish neighbors from an impending roundup by the Germans. When all hiding places appeared to be exhausted, the Danes then turned out in every seagoing vessel that could be found, from ferries and cargo ships to kayaks and rowboats, to ensure that as many Jews as possible could escape the country before the Gestapo closed in.
See also the evacuation from Dunkirk.
The Korean War, when the South was at the brink of being occupied, dozens of nations of the U.N. raced to South Korea's aid, and when tables turn for North Korea China, and the USSR come in to aid it. In essence, this was the U.N. coming to the rescue of South Korea where its predecessor the League of Nations had failed to help Ethiopia in the 1930's.
NATO and the Warsaw Pact were both alliances created to prevent World War III by ensuring that all members would automatically come to the aid of any single member that was attacked. Since the heavy-hitter of each was one of the world's two superpowers at the time, they managed to keep the period of the Cold War relatively peaceful, since no one wanted the Americans or the Soviets bringing armies to their front door.
On 9/11, New York summoned the entire NYPD and FDNY to duty.
In addition, firefighters and other rescue and relief personnel travelled from across the nation to assist in the rescue and recovery efforts in New York City. This included personnel from Oklahoma City who the FDNY had previously helped after the Alfred P. Murrah Federal Building bombing of 1994.
Article 5 of the NATO charter, which states that an attack against one member will be considered an attack against all members, has only been invoked once before, in response to the 9/11 attacks.
In 2012, WABC-TV in New York City was overwhelmed by simultaneously having to cover a presidential election and Hurricane Sandy. They put out a call for help. Because their colleagues all along the eastern seaboard had the same problem, the ABC station in Dallas sent a reporter to New York.
During the American Revolution, the odd-couple pairing of France and America was a major factor in the rebels' victory.