Attempts were made to reunite Europe, ie by Justinian the Great fifty years later, and Charlemagne, King of the Franks, in 800. The latter conquered Germany and Italy, taking the title of Holy Roman Emperor, but his realm was partitioned between three grandsons! Then Charles I, King of Spain, Sicily, Sardinia and Naples tried a completely new method - dynastic marriages which would produce consensual hereditary unions of crowns. Therefore, the Habsburg motto became “Leave the waging of wars to others, but you happy Austria marry!”
King Charles laid the ground by securing his election as Holy Roman Emperor (through bribery of the seven Prince-Electors) in 1519. Then in 1525 he arranged the marriage of his brother, Ferdinand I, to Anna, sister of King Louis II of Hungary in 1521. This resulted in Ferdinand I (brother of Charles) becoming King of Hungary in 1526. That same year, Charles married Isabella, sister of King John III of Portugal. This resulted in his son, Philip II, becoming King of Portugal in 1581. The latter had married Queen Mary I of England in 1554 resulting in him becoming King-Consort. She was, however, barren, so the hoped for heir to the united thrones of Spain and England was never born. Similarly, the marriage of Eleanor, sister of Charles, to King Francis I of France, in 1530 failed to result in dynastic union because Francis remained implacably opposed to the Habsburg domination of Europe.
Charles had thus ensured that his son Philip II became King of Spain and Portugal (and their colonial empires encompassing the whole of Latin America). He had also ensured that his brother, Maximillian I, became King of Hungary and next Holy Roman Emperor thereby uniting those two thrones.
Charles declared: “I speak Spanish to God, Italian to women, French to men and German to my horse!” Philip at Madrid and his uncle, Maximilian at Vienna, maintained Habsburg unity by initiating a policy of intermarriage between their two families. This inbreeding, however, resulted in genetic decline. In 1700, Charles III, King of Spain, Sicily , Sardinia and Naples died impotent and without offspring. The ensuing War of the Spanish Succession resulted in a non-Habsburg (ie the grandson of Louis XIV of France) becoming the next King of Spain and Sicily. The scheme had failed. The interbreeding Habsburg Dynasty merely retained the Holy Roman Empire and Hungary (until 1918). It was a brave attempt:,during the course of the 16th Century there were related Habsburg Monarchs of England, the Holy Roman Empire (Germany), Hungary, Naples, Sicily, Spain and Portugal.
Two further attempts to unite Western Europe were made by Napoleon in the 19th Century and Hitler in the 20th Century. Both were based on wars of conquest and were thus counterproductive and doomed to failure. The European Union may eventually become a United States of Europe, but not in the lifetime of any reader of this blog in 2015!