You may have heard of the "heptarchy" of seven Anglo-Saxon kingdoms. Actually there were about a dozen Anglo-Saxon kingdoms in the year 600, and about five of them were important in Anglo-Saxon history. Some historians have suggested that there might have been hundreds of tiny Anglo-Saxon kingdoms in the 6th century, which merged peacefully or violently to ...
The independent Republics of: Texas, Vermont, and California all peacefully merged into the United States... I don't think independent countries peacefully merging themselves into another country is uncommon.
The gradual merger of Germany into a single country is an example. This took a long time, starting with the absorption of many smaller countries by Prussia, and the formation of customs unions. A major step was the formation of the German Empire in 1871, which created political and administrative integration, and put an end to wars between the German states. ...
Yes, Syria and Egypt merged to one country called United Arab Republic in 1958. They separated again in 1961.
There are many earlier examples:
England and Scotland merged to United Kingdom (1706).
Poland and Lithuania merged in (1569) into the Polish-Lithuanian Commonwealth.