The history of the role of Chargé d’Affairs is tied into the European history of diplomatic ranks. As a system this can get a little bogged down in now outdated details, but the Chargé d’Affaires has actually stayed fairly consistent in its role within the diplomatic system.
As diplomacy became increasingly formalised and complex, battles of prestige began to break out between rival embassies. It was believed that where an ambassador fell in priority reflected their countries place in the world. Things like who entered a room first, became causes of international incidents. One such incident was in 1661, when the delegation of the French Ambassador to England challenged the Spanish delegation over who should march first in the procession to greet the newly appoint Swedish Ambassador. A brawl broke out in the streets of London between the rival embassies. Because both sides held diplomatic immunity, there was little King Charles could do except prevent his own subjects from joining in the fight. To limit such matters, diplomatic rules of priority and diplomatic ranks began to emerge. Ambassadors were widely accepted but not widely used. It was expensive and risky to have a powerful political figure represent your country overseas. As such it was typically restricted to only the countries it was most important to have relations with. E.g. England and France usually exchanged ambassadors as their relations were very important (even when they were at war). Lesser diplomats acquired the titles of agent, resident, envoy, and these carried out single missions such as delivering letters between monarchs, assisting to negotiate treaties, or providing new instructions to the ambassadors.
Towards the latter half of the 18th century, these lesser ranked agents became referred to as Chargé d’Affaires - literally those in Charge of the Affairs of diplomacy. Note that although the term appears in the 1700s, its since been applied to describe diplomats from earlier periods. The reason the name was and is in French is due to French having overtaken Latin as the language of diplomacy in the prior century - it was now quite literal the Lingua Franca.
However, it was still a country-by-country system, and the ranks didn’t always line up. For instance, when Sweden began appointing Minister-Chargés d’Affaires, which baffled most other countries, as they weren’t sure how to treat these new diplomats in their rankings of precedence and prestige.
Eventually, after the defeat of Napoleon, it was determined that an agreed to standard of diplomatic ranks was to be made for the Great Powers of Europe. At the Congress of Vienna, it was agreed that Diplomats would be differentiated into three (soon after made four) ranks. These were: Ambassador Minister/Envoy Minister-Resident (added at the 1818 Congress of Aachen) Chargé d'affaires/Mission head. Soon the smaller states in Europe also began adopting this common standards of ranks. These weren’t the only kinds of diplomats mind you. There were armies of lower functionaries that worked within foreign offices, embassies and consulates. But these formal ranks were now the only diplomats that had the right to lead an embassy.
There were additional rules attached to these ranks. E.g Ambassadors represented the Sovereign, and Ministers represented Governments, and only Ambassadors and Ministers were allowe to be called “Your Excellency”. However, practically an Ambassador and a Minister performed the same functions, with only ceremonial distinctions. Chargés d'affaires, were generally considered to be representing the Foreign Minister. As such they sat at a lower lever of power. This meant that when a country only sent a charge d’affaire, it was typically expected that little significant diplomacy would occur between the two countries.
The receiving country acted reciprocally. A Sovereign received an ambassador, but a Chargés d'affaires was received only by the Foreign Minister. Countries still largely act on the basis of reciprocity, if Country A sends an ambassador to Country B then Country B will reciprocate and send their own Ambassador in turn to Country A. There was prestige politics involved over which countries earned Ambassadors and which only deserved Chargés d’affaires, but this was a matter of choice for each country.
Despite this attempt at regulating diplomats, countries still varied their habits. The British tied the rank to a mission. E.g. You weren’t an Ambassador (E&P) unless you were actually assigned to a British Embassy, and once your appointment to that Embassy ended you were no longer of Ambassadorial rank. The French however appointed agents to the ranks regardless of whether they were assigned to a mission at the time. This sometimes led to a mismatch of the agent’s rank to the rank required for a mission. For example, the French had a Chargé d’Affaires ranked mission in Bavaria, but the post was often actually usually filled by a diplomat who was ranked as a Minister. This mismatch becomes relevant to how the practice developed in later times.
Following two world wars, and significant evolution of diplomatic communication, by the 1960s it was clear that the system designed by Kings and Princes in 1815 was no working in a world increasingly run by Republics. For one thing the 1815 rules weren’t binding on states that weren’t present for the Vienna Congress. This limited the formal system to meaningfully apply to only 7 states by 1950. The International Law Commission proposed a new treaty to regulate diplomatic relations. This draft treaty was adopted in 1961 at the United Nations Conference on Diplomatic Intercourse and Immunities and became one of the most widely adopted treaties in the world. Maddeningly, this new Treaty is called in shorthand the Vienna Convention (also VCDR) because like its forerunner, it too was agreed upon in Austria’s capital. This new treaty maintained the separate Diplomatic ranks, now simplified to: Ambassador Minister Chargé d’Affaires.
The VCDR set out that except for matters of etiquette, these ranks were now functionally identical. As such, Ministers - which were historically used to reflect petty hierarchies and pretensions between states - increasingly became obsolete. Ambassadors became the norm for all state-to-state relations (though for reasons of tradition some countries still call their ambassadors ‘Ministers’).
However, Charges d’Affaires remained as a distinctive rank. It had early-on found a role filling the niche of deputy to the ambassador, and continued to do so. By having a separate and formal rank under the VCDR, the deputy/chargés d’affaires were still able to pursue diplomatic activities independently of the Ambassador when needed.
Importantly, there are two special kinds of Chargé d’Affaires that are in use that do still head missions.
As in previous eras, the appointment of formal ambassadors can be a difficult and labour-intensive act of bureaucracy. It often requires the attention of a country’s head of state who are typically busy with other matters. However with the rise of permanent foreign embassies, a country’s embassy still needs to be able to operate. When there wasn’t an Ambassador currently appointed to a mission or when the Ambassador is otherwise unavailable (such as being summoned home for consultation), a Charge d’Affaires still has the rank and authority to maintain the mission’s ongoing work in the absence of an Ambassador. This function is formally defined under the Vienna Convention as a Chargé d’Affairs ad interim and is the successor to the previous French (and others’) habit of mismatching ranks to missions. It’s also why in French Chargé d’Affaires has also come to mean ‘stand-in’ in a non-diplomatic sense.
The adoption of Chargé d’Affaires for this purpose occurred very early. For example, between 1784 and 1787 there was no French Ambassador to the United States of America and instead matters were handled by a Chargé d’Affaires ad interim who had been the last Ambassador’s Secretary. Such appointment of an interim was and is not an automatic practice, though. A state may still refuse to appoint a Chargé d’Affaires ad interim, even when there is no Ambassador. This is occasionally done for appearance sake. When one is appointed, the practice is for the Ambassador to inform the host country of their own upcoming absence. Thus it is usually the leaving Ambassador that appoints the Chargé d’Affaires ad interim, though sometimes it’s the sending country’s Foreign Ministry directly.
The another type of Chargé d’Affaires still in occasional use is the Chargé d’Affaires en pied (on foot) also known as the Chargé d’Affairs en titre (in title). This is used when a country wishes to maintain diplomatic relations but not at the level of ambassador. This typically reflects displeasure or some other contention between the two countries. E.g. in the 1990s several countries reduced their diplomatic presence in Belgrade to the rank of Chargé d’Affairs en titre to show their displeasure at Yugoslavia’s behaviour in the Yugoslav Wars. Similarly, the People’s Republic of China refused to exchange diplomats of a higher rank than Chargé d’Affaires with Britain until Britain removed the British Ambassador in Taipei.
In short, the role of Chargé d’Affaires has remained remarkably consistent for 250 years. Serving as the lowest rank of the highest class of diplomats, and providing significant flexibility for countries when they pursue their foreign relations.