The essence of both marriage and marriage alliance is to achieve a state of peace through the formation of in laws. The difference between marriage and alliance lies in their status. Marriage is unequal in status, for obvious purposes, to end the war, and is a compromise. The status of the two parties in the marriage alliance is relatively equal, and the power gap is relatively small. Although it also has a purpose, it is more of a cooperative purpose.
Marriage and intermarriage have been effective means of international struggle since ancient times. This method is more effective than sending hostages, but there is also a constraint: the marriage relationship of the ruler cannot be too chaotic. In medieval Europe, conflicts between countries were often resolved through intermarriage, and in the Spring and Autumn Period and Warring States Period of China, intermarriage was also commonly used to resolve conflicts between countries.
The same method was more effective in the Middle Ages of Europe, but almost ineffective in the Spring and Autumn Period and Warring States Period of China. The reason for this is that the aristocratic class in China was accustomed to the insatiable practice of polygamy, which resulted in a decrease in the binding force of marriage. Later on, when a ruler gave his daughter to another ruler, it was like giving an ordinary gift. He didn't think it was expensive to hold it himself, and others didn't think it was expensive. If this daughter has some beauty, perhaps she can also play a political role, otherwise, giving her away is like not giving her away.