Which state had the most successful territorial expansion?

The most successful territorial expansion was achieved by the Russian Empire.

The Russian Empire, spanning from the 16th to the early 20th century, had the most successful territorial expansion in history. At its peak, it was the third largest empire in history, extending across Eurasia from Eastern Europe to the Arctic Ocean and from the Baltic Sea to the Pacific. This vast expansion was achieved through a combination of military conquests, treaties, and strategic marriages.

The expansion began under the reign of Ivan IV, also known as Ivan the Terrible, who established the Tsardom of Russia in 1547. He expanded the nation's borders eastward into Siberia, laying the groundwork for future expansion. However, the most significant period of territorial growth occurred during the reign of Peter the Great in the late 17th and early 18th centuries. Peter modernised the Russian army and navy, enabling more effective military campaigns. He also founded the city of Saint Petersburg, which became Russia's new capital and a symbol of the country's westernisation.

Under Catherine the Great in the late 18th century, Russia continued to expand, primarily at the expense of the Ottoman Empire and Poland. The partitions of Poland between 1772 and 1795 resulted in Russia acquiring a significant portion of Polish territory. Meanwhile, a series of wars against the Ottoman Empire led to gains in the Black Sea region.

The 19th century saw further expansion into Central Asia, culminating in the conquest of Turkestan. The Russian Empire also expanded into the Far East, gaining territories such as Sakhalin and the Kuril Islands. This expansion was driven by a desire for resources, trade routes, and strategic advantages over other powers.

In conclusion, the Russian Empire's territorial expansion was unparalleled, resulting in a vast empire that spanned three continents. This was achieved through a combination of military conquests, diplomatic manoeuvres, and strategic planning. The empire's expansion had profound impacts on the regions it incorporated, influencing their cultural, political, and economic development.