Hellenistic Diplomacy

Hellenistic diplomacy, like all Hellenistic rulership, was fundamentally personal: agreements and alliances were not struck between abstract states or nations, but between individuals. When a monarch died, previous agreements no longer stood, and new agreements with the next monarch had to be negotiated. The contemporary purpose of these marriage alliances at the time was to create a direct and personal link two monarchs, symbolized and meditated by marriage to a royal woman. In other words, the marriage of Stratonice I to Seleucus created a direct bond between her husband and her father that, in turn, united their two families.
Alex McAuley, “Weaving the Dynastic Web: Hellenistic Marriage Alliances,” Ancient History 49 p. 22 https://www.karwansaraypublishers.com/en-ca/products/ancient-history-magazine-49
Ten years ago Canadian Armed Forces members had to take an oath or affirmation to “Elizabeth II the Queen of Canada, her heirs and successors according to law” (the exact wording may be different). Getting people to pledge loyalty to an abstract monarchy rather than a specific king or queen is really unusual in world history.