Military district
In 1921, due to reorganization, the military districts were replaced with Dowództwo Okręgu Korpusu (DOK – Corps District Command). In the Second Polish Republic there were ten DOKs: 

I – Warszawa
II – Lublin
III – Grodno
IV – Łódź
V – Kraków
VI – Lwów
VII – Poznań
VIII – Toruń
IX – Brześć nad Bugiem
X – Przemyśl
Each DOK consisted of four large units (three infantry divisions and one cavalry brigade).
For district arrangements after World War II see Polish Land Forces. The Kraków Military District disbanded in 1953. From 1999 Poland has been divided into two military districts, the Pomeranian Military District and the Silesian Military District, both were disbanded by the end of 2011.

Russia: 

Russian Empire: The Russian Empire's military district (Russian: вое́нный о́круг, voyenny okrug) was a territorial association of military units, formations, military schools, and various local military establishments.  This territorial division type was utilized in Imperial Russia, USSR and is currently in use in Russian Federation.
Such territorial division provided convenient management of army units, their training and other activities regarding the country's readiness to defend itself.

Soviet Union: In the USSR, the military districts continued to perform the same role they had done in the Russian Empire, with first six military districts (Yaroslavsky, Moskovsky, Orlovsky, Belomorsky, Uralsky, and Privolzhsky) were formed on 31 March 1918 during the Russian Civil War.
This increased to 17 military districts of the USSR at the beginning of July 1940 shortly before the USSR was invaded by Germany and entered the Second World War, and were used to create combat Fronts after commencement of the German invasion of the USSR.
During the war the districts were further divided into geographic regions for logistic reasons, these being:

North and North Western districts
West and Central USSR districts
South and South Western districts
Siberian and Central Asian districts
Far Eastern districts
After the war, the number was increased to 33 to aid in demobilisation of forces, but by October 1946, they had been reduced to 21.
By the end of the 1980s, immediately before the dissolution of the Soviet Union, there were sixteen military districts, within three to five main strategic Theatre groupings.

Russian Federation: Military districts (Russian: вое́нный о́круг, voyenny okrug) in the Russian Federation operates under the command of the district headquarters, headed by the district commander, and is subordinated to the General Staff of the Armed Forces of the Russian Federation. (Previously under Commander-in-Chief of the Ground Forces General Nikolai Kormiltsev, the military districts reported to the General Staff via the Russian Ground Forces staff.) It is a territorial association of military units, formations, military schools, and various local military establishments. This territorial division type was historically adopted, originally by Imperial Russia, to provide a more efficient management of army units, their training and other operations activities related to combat readiness.

From 1992 to 2010, the Armed Forces maintained a diminishing number of former Soviet Armed Forces districts – Leningrad Military District, Moscow Military District, Volga-Urals Military District, North Caucasus Military District, Siberian Military District, Far East Military District.

In 2009–2010, these districts were reorganised into 4 Military Districts comprising regional Joint Strategic Commands.

In 2014 Northern Fleet was reorganized into separate Joint Strategic Command.

Leningrad Military District with headquarters in Saint Petersburg
Southern Military District with headquarters in Rostov-on-Don
Central Military District with headquarters in Ekaterinburg
Eastern Military District with headquarters in Khabarovsk
Moscow Military District with headquarters in Moscow

Sweden: The military district (Swedish: Militärområde, usually abbreviated to Milo) was an administrative division of the Swedish Armed Forces, and was a higher regional level subdivision. The commander of a military district, the Militärområdesbefälhavare (also militärbefälhavare), commanded the Swedish Army divisions stationed in the region, the regional naval command, the regional air defence sector as well as the lower regional level subdivision defence districts that made up the military district. The commander answered directly to the Supreme Commander. The military districts in the modern form were created in 1966, and each district was named according to the geographical area they covered. Several changes were made, such as creating or merging districts, until all military districts were disbanded in 2000. After the Defence Act of 2000 the military districts were replaced by new military districts (Swedish: Militärdistrikt, usually abbreviated to MD). The new military districts corresponded geographically to the former military districts, however, they did not have the same territorial and operational tasks. In 2005, the military districts were replaced to some extent by four Security and Cooperation Sections (Swedish: Säkerhets- och samverkanssektioner).

United Kingdom: British Army regional districts have evolved slowly over the previous 150 years or so. For many years there were regional commands in the UK, including Aldershot Command (from 1880), Eastern Command, Northern Command, Scottish Command, Southern Command and Western Command (from 1905). By 1985 these were superseded by districts, and until the spring of 1991 there were nine of them. Antony Beevor wrote in his revised edition of Inside the British Army in 1991 that '..the first of the minor districts to be amalgamated were North West District, Western District, and Wales, to form a new Western District.' HQ Northern Ireland remained separate and reported to Headquarters United Kingdom Land Forces only on non-operational matters.

From 1995, UK commands and later districts were replaced by regenerative divisions. 2nd Division, 4th Division, 5th Division and London District acted as regional commands within the UK reporting to Commander Regional Forces. Scotland District was absorbed by 2nd Division in 2000. The divisions were responsible for training subordinate formations and units under their command for operations in the UK, such as Military Aid to the Civil Community, as well as training units for overseas deployments. 2nd, 4th and 5th Divisions were replaced by Support Command on 1 November 2011.

United States: The military department was a military and administrative command of the US Army.

Present day US military organization is structured around Unified Combatant Commands, which encompass different geographical areas and responsibilities.

United States Africa Command
United States Central Command
United States European Command
United States Indo-Pacific Command
United States Northern Command
United States Southern Command

Uzbekistan: Military districts (Uzbek: Harbiy okruglar) of the Armed Forces of Uzbekistan are under the jurisdiction of the defense ministry of the republic. In May 2001, the Tashkent Garrison was transformed into the Tashkent Military District. The following are a list of military districts in Uzbekistan:

Northwest Military District (HQ Nukus)
Southwest Special Military District (HQ Qarshi)
Central Military District (HQ Dzhizak)
Eastern Military District (HQ Ferghana)
Tashkent Military District (HQ Tashkent)
The replenishment of the ranks of divisions and units in wartime is done by order of the commanders of military districts.

Vietnam: Vietnam People's Army has 8 Military Regions:

Hanoi Capital Command: It is directly under the Ministry of Defense of Vietnam; tasked to organize, build, manage and command armed forces defending the capital. The headquarters is in Hanoi.
1st Military Region: It is directly under the Ministry of Defense of Vietnam; tasked to protect against foreign invaders; and to organize, build, manage and command forces in northeastern Vietnam. The headquarters is in Thai Nguyen.
2nd Military Region: It is directly under the Ministry of Defense of Vietnam; tasked to organize, build, manage and command armed forces defending northwestern Vietnam. The headquarters is in Viet Tri.
3rd Military Region: It is directly under the Ministry of Defense of Vietnam; tasked to organize, build, manage and command armed forces defending the Red River Delta area. The headquarters is in Hai Phong.
4th Military Region: It is directly under the Ministry of Defense of Vietnam; tasked to organize, build, manage and command armed forces defending north central Vietnam. The headquarters is in Vinh.
5th Military Region: It is directly under the Ministry of Defense of Vietnam; tasked to organize, build, manage and command armed forces defending south central Vietnam, including the western highlands and south central coastal provinces. The headquarters is in Da Nang.
7th Military Region: It is directly under the Ministry of Defense of Vietnam; tasked to organize, build, manage and command armed forces defending southeastern Vietnam. The headquarters is in Ho Chi Minh City.
9th Military Region: It is directly under the Ministry of Defense of Vietnam; tasked to organize, build, manage and command armed forces defending the Mekong Delta. The headquarters is in Cần Thơ
The Army of the Republic of Vietnam originally had four corps, for example I Corps (South Vietnam). Later they were redesignated Military Regions 1–4.

See also: Unified Combatant Command
United States historical military districts
Fortified district

References: 
Beevor, Antony (1991).