What changes took place in international relations in the 19th century. what are the reasons for these changes?

For most of the 19th century, Europe was in the process of creating two states – Italy and Germany. They took shape by 1870 and by the end of the century began colonial expansion, for example, in Africa (Namibia, Tanzania and Cameroon – Germany, and Eritrea and Somalia – to Italy).

By the end of the century, two European military alliances had developed:

Entente. Russia and France since 1893 and at the beginning of the 20th century, Great Britain joined it.
Central Powers: Italy, Austria, Germany.
By 1878, several independent states appeared in the Balkans (Montenegro, Serbia, Greece, Romania, Bulgaria). Despite the role of Russia in their struggle for independence, some of them, from 1885, began to pursue a pro-Austrian and pro-German course (Bulgaria).

The Ottoman Empire weakened throughout the 19th century. She lost the wars with Russia in 1806-1812 and 1828-1829, and in 1853-1856 and 1877-1878 she was saved from complete defeat by France and England.

In Asia, the British throughout the 19th century pursued a policy of pressure on China (the Qing Empire), and from 1895 Japan also won the first war with it.