“Solomon made an alliance with Pharaoh king of Egypt and married his daughter. He brought her to the City of David until he finished building his palace and the temple of the LORD, and the wall around Jerusalem.”
—1 Kings 3:1, The Bible
It is said that nations do not form friendships but alliances: sometimes the alliances are without apparent rational and appear incongruent, such as the Marxist-Islamic alliance; other times they make sense, such as the alliance between western liberal democracies and the military alliance forming the basis of NATO. Alliances are not friendships, although nations can have long-standing friendly relations based on the necessities of trade and commerce and mutual defense against a common enemy or threat; the defense of shared and common interests, such as the maintenance of open trade, is often how alliances are initially formed.
Whether such arrangements are always maintained is often based on many factors, necessity and long-term strategic interests being important considerations for heads of state. How and why alliances are formed initially is a field of study that can include political scientists, historians and military experts, and religious-studies scholars in particular if we are looking at ancient treaties. Marriage was a common way to form alliances between nations.
In the pre-modern age, for example, kings and queens married to cement an alliance between kingdoms. It is said that King Solomon, for example, married many of his wives for such political reasons, often entering into marriages with his enemies; love was hardly the reason. The biblical account said that he had 700 wives and 300 concubines; that's a lot of political alliances. According to the bible, and the Jewish commentators writing about the period of the Jewish monarchs, these alliances eventually became problematic:
King Solomon, however, loved many foreign women besides Pharaoh’s daughter—Moabites, Ammonites, Edomites, Sidonians and Hittites. They were from nations about which the Lord had told the Israelites, “You must not intermarry with them, because they will surely turn your hearts after their gods.” Nevertheless, Solomon held fast to them in love. He had seven hundred wives of royal birth and three hundred concubines, and his wives led him astray. As Solomon grew old, his wives turned his heart after other gods, and his heart was not fully devoted to the Lord his God, as the heart of David his father had been. He followed Ashtoreth the goddess of the Sidonians, and Molek the detestable god of the Ammonites. So Solomon did evil in the eyes of the Lord; he did not follow the Lord completely, as David his father had done.
—1 Kings 11:1-6
Apparently, the many wives and their foreign traditions had undue influence on Solomon, to the detriment of Israel's declared dedication to its sole and jealous god; such was the thinking of the prophets, who subsequently judged Solomon's failures in the harsh light of devotion to a singular idea. Yet such political alliances, often through marriage, continued—with the best of intentions and reasons. During the middle ages and later, for example, kings and queens continued the same marital and political alliances, with court officials continuing to remain suspicious of outside or foreign influences. Marriage between rival monarchies mixed the two ruling houses, and thus the two bloodlines.
Notable examples include the mythological marriage of Helen of Troy and Menelaus of Sparta during ancient Greece's Bronze Age; Catherine the Great of Germany and Peter III of Russia in the early 18th century; Queen Victoria of England and Prince Albert of Saxe-Coburg, Germany, in the mid-19th century; and the modern example of Olav V of Norway and Princess Märtha of Sweden in 1929; Juan Carlos I of Spain and Princess Sophia of Greece in 1962; and Constantine II of Greece and Princess Anne Marie of Denmark in 1964. There are many more examples, thus making it hard to prove the claim that someone's blood is pure of any foreign "contamination"—a specious argument put forth by nationalists and extremists.
While today such arranged marriages are not the norm, as monarchies do not hold the same power in general that they once did, it is still done among religious sects, including prominent Jewish Hasidic families, where leaders of long-term Hasidic dynasties arrange marriages for reasons other than romantic love. Perhaps the solution to the current and continuing conflict among nations is to return to such political alliances through marriage.