So this actually came up in an interview of mine recently, and I was initially taken aback (perhaps wrongly) that I got the impression the academic interviewing me did not seem to consider Leopold II's 'Congo Free State' as an example of imperialism.
I've discussed this with several (non-historians) and they have all tended to conclude it was an example of imperialism. Here are a couple of points raised in favour:
Several points against:
Again, just chucking around ideas, any comprehensive answer from someone far more knowledgeable than myself would be appreciated ahahaha
In any contemporary conversation, someone who tries to exclude the Congo Free State from a definition of imperialism is at best intellectually mischievious or is weirdly anachronistic in their thinking.
E.g., in the era of the "new imperialism" of the late 19th Century, legalistic distinctions between different kinds of territorial sovereignty by European interests were of prime importance to the European powers gathered around various negotiating tables, most prominently the Berlin Conference of 1884-85. A certain kind of political-diplomatic history of European imperialism in Africa retained a kind of technical concern for the difference between different sorts of legalistic distinctions between types or modes of colonial authority into the 1950s-1960s, but this quickly became seen (justifiably) as an irrelevance on multiple levels. In the case of Leopold's Congo, the distinction was ideologically important to the British, German and French governments in the aftermath of revelations about the abuses taking place in the Congo Free State, as a way to explain (and isolate) what was happening there--that the Congo Free State wasn't "normal" colonial rule but instead an aberrant kind of personal fiefdom, hence the abuses. This, among other things, helped the French government turn attention from more or less the exact same practices of rubber extraction in its holdings in Central Africa, and the German government deflect concerns about its genocide of the Herero people in South-West Africa.
This is also what led the British government to push for the eliminate of "company rule" in the territories controlled by the British South Africa Company (present-day Zimbabwe, Zambia and Malawi), on the grounds that the era of company sovereignty had passed. But in practice, there was little difference between the regime of Leopold (and other chartered companies) in the Congo Free State and extractive regimes in early colonial Africa elsewhere. Moreover, Leopold was deeply involved in the process of the "Scramble for Africa"--arguably he was one of its key instigators and conceptual authors, and that is the heart of imperialism as a process in the late 19th Century. In every respect that matters, the Congo Free State was an exemplar of 19th Century European imperialism, not its exception.
/u/swarthmoreburke gave a very good answer already, and I agree that the Congo Free State is widely considered as part of the New Imperialism.
I just want to cover a few bits of errata that I think help to contextualize and strengthen the argument that Congo Free State was of-a-piece with other, earlier examples (17th-19th c.) of European imperialism as well as contemporary New Imperialism of the late 19th c.
Not directly owned by Belgium until 1908, completely independent project of Leopold II
This statement is true, but there is more to the story. In the late 1880s up to 1906, the Parliament of Belgium had repeatedly issued loans to the Congo Free State amounting to tens of millions of Belgian francs. In 1889, when asking for a particularly large loan of 25 million francs, Leopold II submitted a letter and his will to parliament, announcing his intention to give the Congo Free State as inheritance to Belgium (i.e. CFS becomes a Belgian colony on his death), but also announcing that he was amenable to Belgium annexing and turning CFS into a Belgian colony before his death.^1 As part of issuing the loan, the Belgian parliament insisted on language that demanded repayment after 10 years and allowed annexation of the Congo Free State in 1901 if CFS was in default.
Now, in parliamentary debates in 1906, socialist parliamentarians made complaints that in the 1890s and early 1900s, Leopold II had publicly extolled the CFS as a Belgian colony-in-waiting and a great Belgian project, while at the same time brushing back parliamentary questions about CFS finances by claiming that it was a sovereign state not answerable to the Belgian parliament.^2 The debate treated the agreement of 1890 around the loan, and the will, as essential instruments that allowed annexation of the Congo Free State.
So, the long and the short of it is that Belgium was deeply financially invested in the Congo Free State, and Leopold II clearly sought to make Belgians see the CFS as a colony-in-waiting through his gesture in 1889. Though, a cynical reader may question his motives, given that Belgian loans to CFS for capital investment would benefit him financially (allowing greater transportation and export of resources). And worth noting, Leopold II also sought loans internationally, and was forced to offer France the opportunity to annex in case of default.
Additionally, we should note that Leopold II had sought to present the efforts (to UK, France, Germany, the US) of the Association International Africaine from 1871-1885 not as building a Belgian colony but instead as a non-governmental, international effort that would allow free trade in the Congo basin. That is, he was playing on great power competition, and presenting the A.I.A./CFS as a non-partisan, non-national alternative to German, French, Portuguese and British competition.
Imperialism defined as: ' a policy of extending a country's power and influence through colonization, use of military force, or other means' - was not intended for the furthering of Belgium, but rather Leopold (possibly could argue that the eventual transfer of the Belgian Congo to Belgium following international outcry was just shifting from one form of imperialism to another)
So, my above comments give a confused picture, but I think it is fair to conclude that Leopold wished to promote the impression to Belgians that the Congo Free State would increase their national prestige, wealth and power (even if his real motives were personal wealth and power).
Also, /u/Swarthmoreburke's mention of Cecil Rhodes British South Africa Company is quite apt. The establishment of companies as an instrument of colonial expansion was fairly common in the 1870s-1900. Aside from the Association International Africaine and the BSAC, there were British companies like the Royal Niger Company the German East Africa Company and French Senegal company and Equatorial African Association which were all chartered companies established to extend British, German or French influence (respectively) and pursuing commercial opportunities in Africa while sparing the metropolitan governments a large portion of the expense of conquest and administration.
Of course, the major difference was that the Association International Africaine and the later Association International du Congo were presented as international organizations, and did not publicly present their goals as benefiting Belgium (or Leopold II personally). On the other hand, after Leopolds claims were ratified by the Berlin conference in 1885, he changed his tune and presented the Congo Free State as a Belgian colony-in-waiting in 1889 (as covered above).
We can also look at earlier examples of corporate colonial ventures like British East India Company's progressive conquest and rule of the subcontinent from 1770s to 1858, when the company's colony was nationalized by the United Kingdom.
Or the Dutch East Indies Company's conquest and administration of what is now Indonesia from the 1500s to 1800s, when the company's colonies were nationalized by the Dutch republic.
So, the commercial nature of the AIA/AIC/CFS does not set it totally apart from contemporary or earlier colonial ventures. And whether Leopold envisioned the CFS as primarily extending Belgian or his own personal influence is ambiguous, I feel.
1 "the Congo Free State and Congo Belge" by Frederick Starr in The Journal of Race Development vol 1, no 4, 1911. page 391.
2 Verbatim Report of the Five Days Congo Debate in the Belgian House of Representatives trans. by E.D. Morel, 1906. pp 28, pp 70.
Edit- To add one more source that addresses the ambiguity of Leopold's thinking regarding colonial policy, check out King Leopold's Imperialism and the Origins of the Belgian Colonial Party; 1860-1905 by Vincent Viaene, in The Journal of Modern History vol 80, no 4 (december 2008). pp 741-790. This one is a bit dense with historiography of Leopold and the Colonial party, but I thought it irresponsible to lead off with a source from 1911 from the Journal of Race Development without more modern scholarship to provide an antidote to the imperial optimism of Starr.