In general, expansionism consists of policies of governments and states that involve territorial, military or economic expansion. While some have linked the term to promoting economic growth (in contrast to no growth or sustainable policies), more commonly expansionism refers to the doctrine of a state expanding its territorial base or economic influence. This occurs usually, though not necessarily, by means of military aggression. Compare empire-building, colonialism, and mensurable.
Anarchism, reunification or pan-nationalism are sometimes used to justify and legitimize expansionism, but only when the explicit goal is to reconquer territories that have been lost, or to take over ancestral lands. A simple territorial dispute, such as a border dispute, is not usually referred to as expansionism.
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The militarist and nationalistic reign of Czar Nicholas I (1825-55) led to wars of conquest against Persia, 1826, and Turkey, 1828-9. Various rebel tribes in the Caucasus region were crushed. A Polish revolt in 1833 was ruthlessly crushed. Russian troops in 1848 crossed into Austria-Hungary to put down the Hungarian revolt. Russification policies were implemented to weaken minority ethnic groups. Nicholas also built the Kremlin palace and a new cathedral in St Petersburg. But Pan-slavism ambition led to further war with Turkey (the Sick man of Europe) in 1853 provoked Britain and France into invading Crimea, and Nicholas died, supposedly of grief at his defeat. 
The German Second Reich (1871-1918) underwent an industrial revolution under Bismarck, who also reformed and expanded the army. Poles and Catholics were persecuted. Colonies were acquired in Africa and China. In 1890, Kaiser Wilhelm II dismissed Bismarck and resolved to build a world-class Navy, which led to an arms race with Britain and thence to World War One. 
From 1933 the Third Reich under Hitler laid claim to the Rhineland, the Sudetenland, unification, Anschluss, with Austria in 1938, and the whole of the Czech lands the following year. After war broke out, Hitler and Stalin divided Poland between Germany and the USSR. In a Drang nach osten aimed at achieving Lebensraum for the German people, Germany invaded the USSR in 1941. 
Colonialism, a form of expansionism is the policy of a nation seeking to extend or retain its authority over other people or territories, generally with the aim of developing or exploiting them to the benefit of the colonizing country. The European colonial period was the era from the 15th century to the mid-20th century when several European powers had established colonies in the Americas, Africa, and Asia.
Expansionist nationalism is an aggressive and radical form of nationalism that incorporates autonomous, patriotic sentiments with a belief in expansionism. The term was coined during the late nineteenth century as European powers indulged in the 'Scramble for Africa' in the name of national glory, but has been most associated with militarist governments during the 20th century including Fascist Italy, Nazi Germany, the Japanese empire, and the Balkans countries of Albania (Greater Albania), Bulgaria (Greater Bulgaria), Croatia (Greater Croatia), Hungary (Greater Hungary), Romania (Greater Romania) and Serbia (Greater Serbia).
Nations seeking to expand at present include Russia, which some say, threatens NATO's existence.  Iran, the largest Shi'ite state, has extended its influence in Iraq, Lebanon, Syria, Yemen and Afghanistan.  China is expanding its operations and influence in the South China Sea, claiming possession of disputed offshore islands in the search for oil and gas. 
In the nineteenth century, theories of racial unity such as Pan-Germanism, Pan-Slavism, Pan-Turkism and the related Turanism, evolved. In each case, the dominant nation (respectively, Prussia, Russia and the Ottoman Empire, especially under Enver Pasha,) used these theories to legitimise their expansionist policies.
George Orwell's satirical novel Animal Farm is a fictional depiction, based on Stalin's USSR, of a new elite seizing power, establishing new rules and hierarchies, then expanding economically while compromising their ideals; while Robert Erskine Childers in The Riddle of the Sands portrayed the threatening nature of the German Second Reich. Elspeth Huxley's novel Red Strangers shows the effects on local culture of colonial expansion into sub-Saharan Africa.
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