Short answers, pulled from the story.
Diplomacy is the communication by representatives of state, intergovernmental, or non-governmental institutions intended to influence events in the international system. It is the main instrument of foreign policy, and most international treaties, agreements, and alliances result from diplomatic negotiations.
The term diplomacy derives from the 18th-century French word diplomate, based on the ancient Greek diploma, meaning roughly an object folded in two. It reflected the practice of sovereigns handing out a folded document to confer an official privilege, since folding protected the contents before the envelope existed.
Some of the earliest diplomatic records are the Amarna letters from the 14th century BC, written between Egyptian pharaohs and the Amurru rulers of Canaan. Peace treaties were also concluded between the Mesopotamian city-states of Lagash and Umma around 2100 BC, and the Egyptian-Hittite peace treaty followed the Battle of Kadesh in 1274 BC.
The 1961 Vienna Convention on Diplomatic Relations is a framework, ratified by most of the world's sovereign states, that governs diplomatic procedures, methods, and conduct. It formalized diplomatic rights first established in mid-17th-century Europe and protects diplomats from being persecuted or prosecuted while on a mission.
The Arthashastra is the oldest treatise on statecraft and diplomacy, attributed to Kautilya, also known as Chanakya, the principal adviser to Chandragupta Maurya. It presents the rajmandala model of twelve surrounding states and four policy approaches: conciliation, gifts, rupture or dissent, and force, naming war the last resort.
Types of diplomacy include appeasement, counterinsurgency, debt-trap, economic, gunboat, hostage, humanitarian, migration, nuclear, preventive, public, quiet, science, soft power, and city diplomacy. Each carries its own advantages and disadvantages, ranging from coercive gunboat displays to the cultural influence of soft power, which Joseph Nye defined.
Diplomatic immunity protects diplomats from being persecuted or prosecuted while on a diplomatic mission, a principle formalized by the 1961 Vienna Convention on Diplomatic Relations. A diplomat who commits a serious crime may be declared persona non grata, meaning an unwanted person, and is then often tried in their homeland.