- Under Secretary of Defense for Acquisition and Sustainment
- Under Secretary of Defense for Intelligence
- Under Secretary of Defense for Personnel and Readiness
- Under Secretary of Defense for Policy
- Under Secretary of Defense for Research and Engineering
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- Under Secretary of the Navy
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- Underwater Archaeology Branch, Naval History & Heritage Command
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- Underwater Defence Group Command
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- Underwater Demolition Badge
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- Underwater Demolition Team
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- Underwater Offence Group Command
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- Unditching roller
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- Unha
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- Unidentified body on Christmas Island
- Uniejów Castle
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- Union Soldiers and Sailors Monument
- Union Street drill hall, Keith, Moray
- Union blockade
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- Union of Active Struggle
- Union of Armed Struggle
- Union of Arras
- Union of Brussels
- Union of Cities
- Union of Congolese Patriots
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- Union of Forces for Democracy and Development–Fundamental
- Union of Grodno
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- Union of Grodno (1566)
- Union of Lithuanian Freedom Fighters
- Union of Lublin
- Union of Mujahadin
- Union of Patriotic Youth
- Union of Resistance Forces
- Union of Retaliation
- Union of South Africa Commemoration Medal
- Union of South Africa King's Medal for Bravery
- Union of South Africa King's Medal for Bravery, Gold
- Union of South Africa King's Medal for Bravery, Silver
- Union of South Africa Queen's Medal for Bravery, Gold
- Union of South Africa Queen's Medal for Bravery, Silver
- Union of Transylvania with Romania
- Union of Utrecht
- Union of the Committees of Soldiers' Mothers of Russia
- Union sacrée
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- Unit 124
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- Unit 9420
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- Unit Citation for Gallantry
- Unit Colour Patch
- Unit Deployment Program
- Unit Egoz
- Unit Ei 1644
- Unit Identification Code
- Unit Interventie Mariniers
- Unit Tindakan Cepat
- Unit citation
- Unit cohesion
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- Unit run
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- Unitas Medal
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- United Nations Mission for Justice Support in Haiti
- United Nations Mission for the Referendum in Western Sahara
- United Nations Mission in Bosnia and Herzegovina
- United Nations Mission in Côte d'Ivoire
- United Nations Mission in East Timor
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