In many wars, huge numbers of soldiers have died without their remains being identified. In modern times the practice developed for nations to have a symbolic Tomb of the Unknown Soldier that represented the war grave of those unidentified soldiers. They usually contain the remains of a dead soldier who is unidentified (or "known but to God" as the stone is sometimes inscribed), and is thought to be impossible to ever identify. Much work goes into trying to find a certain soldier, and to verify that it is indeed one of the relevant nation's soldiers.
Perhaps the first memorial of this kind in the world is the 1849 Landsoldaten ("Foot Soldier") monument of the First War of Schleswig in Fredericia, Denmark. Another early memorial of this kind is the 1866 memorial to the unknown dead of the American Civil War.
The current trend was started by the United Kingdom when, following World War I, it first buried an Unknown Warrior on behalf of all British Empire Forces in Westminster Abbey in 1920, leading other nations to follow their example. The most famous tomb is that in France under the Arc de Triomphe that was installed in 1921 honouring the unknown dead of the First World War.
These tombs are also used to commemorate the unidentified fallen of later wars. Although monuments have been built as recently as 1982 in the case of Iraq, it is unlikely that any further ones will be constructed. Advances in DNA technology mean that even the tiniest fragment of bone is usually identifiable.
National examples:
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Argentina: the Tomb of the Unknown Soldier of the Independence, at the Metropolitan Cathedral in Buenos Aires.
Australia: the Tomb of the Unknown Soldier at the Australian War Memorial in Canberra
Austria: the Tomb of the Unknown Soldier at the Heldenplatz in Vienna
Belgium: the Tomb of the Unknown Soldier at the base of the Colonnade of the Congress in Brussels
Brazil: the Tomb of the Unknown Soldier at the platform of the Monument to the Pracinhas in Rio de Janeiro
Canada: the Tomb of the Unknown Soldier at the National War Memorial, Confederation Square, in Ottawa
Chile: the Tomb of the Unknown Soldier in the General Manuel Baquedano Square in Santiago. It contains the remains of a soldier who died in 1881 during the War of the Pacific.
Czech Republic: the Tomb of the Unknown Soldier at the National Memorial on Žižkov (Vítkov) Hill in Prague
Denmark: the Landsoldaten ("The Foot Soldier") monument in Fredericia — perhaps the first monument of this kind in the world
Egypt: The Tomb of the Unknown Soldier is in Cairo, also including president Anwar Sadat tomb.
Finland: the Tomb of the Unknown Soldier in the Hietaniemi military cemetery in Helsinki
France: the Tomb of the Unknown Soldier beneath the Arc de Triomphe in Paris
Germany: the Tomb of the Unknown Soldier is a former 19th century guard house, called Neue Wache in the street Unter den Linden, Berlin
Greece: The Monument of the Unknown Soldier (see photo) is in Syntagma Square (Constitution Square), Athens
Iraq: The Monument of the Unknown Soldier is in Baghdad
India: The Tomb of the Unknown Soldier is under The All-India War Memorial Arch, New Delhi
Indonesia: The Field of Honour is a memorial to unknown soldiers, and is in Bandung. There is also the Tomb of the Unknown (Dutch) Sailor in Kembang Kuning war cemetery in Surabaya.
Italy: the Tomb of the Milite Ignoto is in Rome, Piazza Venezia, in the Altare della Patria (Vittoriano)
New Zealand: the Tomb of the Unknown Warrior is at the National War Memorial in Wellington
Perú: the Tomb of the Unknown Soldier in the Plaza Bolivar (Bolivar Square) in Lima. It contains the remains of a soldier who died in 1881 during the War of the Pacific.
Philippines: the Tomb of the Unknown Soldier in the Libingan ng mga Bayani (Cemetery of Heroes) in Manila
Poland: the Tomb of the Unknown Soldier is in Marshall Józef Piłsudski Square in Warsaw
Portugal: the Tomb of the Unknown Soldier is in the Mosteiro da Batalha.
Romania: the Tomb of the Unknown Soldier is in Carol Park, Bucharest
Russia: the Tomb of the Unknown Soldier is in Alexander Park, Moscow
Serbia: the Monument to the Unknown Hero is on Mt. Avala, Belgrade
Slovenia: the Monument to the Unknown French Soldier is in French Revolution Square in Ljubljana
Ukraine: the Tomb of the Unknown Soldier is in Dnieper Park, Kiev
United Kingdom: the Tomb of the Unknown Warrior at Westminster Abbey in London
United States: the Tomb of the Unknowns at Arlington National Cemetery, Virginia
United States: the Tomb of the Unknown Soldier at Washington Square in Philadelphia, Pennsylvania