The Austro-Hungarian Empire tried to make Serbia a client state in order to form a Christian opposition to the Ottoman Empire. That changed after a revolution in Serbia in 1900. Serbia now came under Russian protection, which was forming a pan-Orthodox opposition to the Latin Christianity represented by the Austro-Hungarian empire. In 1914, Russia repeatedly warned the Austro-Hungarian Empire against attacking Serbia. When it did attack, Russia mobilized its army. Russia also wanted Bulgaria and Montenegro as client states.
Great Britain and Austria both considered Serbia as a client state controlled by Russia. Most historians call Serbia a client state but historian Christopher Clark disagrees. He says the Russians made a mistake in thinking Serbia was a client state. In an unpublished commentary Clark argues:
"It was a risk enhancing initiative [of Russian Foreign Minister Serge Sazanov] to allow Serbia to become to see Serbia as a kind of client; ... Serbia, to my knowledge, has never been a client of anyone. [...] This is a mistake, when Great Powers think they can secure the services of "client states"; That Those "clients" are never in fact "clients"! That's a mistake that is presumably going to continue being made by our political leaderships, though one hopes one day it will stop."
In the 20th century, France started to apply the concept of Françafrique, its name for its former African colonies, sometimes extended to the former Belgian colonies. At present the term is used on some occasions to criticise the allegedly neocolonial relationship France has with its former colonies in Africa.
The countries involved provide oil and minerals important to the French economy. In addition, French companies have commercial interests in several countries of the continent. As if that were not enough, Francophone countries in Africa help to sustain the image of France as a world power, by giving votes of support for French initiatives at the UN.
In the late 19th century, the Japanese Empire gradually reduced Joseon Korea's status to that of a client state. In the early 20th century, this was converted to direct rule. Manchukuo, in contrast, remained a puppet state throughout World War II.
Nauru has been regarded as a client state of Australia by some sources, as it is heavily dependent on economic support from Australia, uses Australia's currency and processes and houses unauthorised asylum seeker arrivals to Australia under the Pacific Solution. In The Guardian, Ben Doherty wrote that "Nauru is a client state in every sense, kept functioning, just, by its wealthy neighbour. But its dependence on Australian largesse makes its government entirely beholden to its benefactor's interests, even at the expense of its own people" and described Nauru as a "tiny, impoverished client state in the middle of the Pacific". Refugee advocate David Manne labelled a plan by Nauru to sign the 1951 Refugee Convention as a "cynical marketing tool" by a "client state of Australia".
Sierra Leone - This is because of British intervention in Sierra Leone in 2000. This was a humanitarian intervention where Britain saved the government of Sierra Leone from collapse. Britain then helped Sierra Leone rebuild after with military, political and financial support. 
^Collected studies: Alexander and his successors in Macedonia, by Nicholas Geoffrey Lemprière Hammond,1994,page 257,"to Demetrius of Pharos, whom she set up as a client king
^Russian Foreign Minister Sergey Sazonov warned Austria in 1914 that Russia "Would respond militarily to any action against the client state." Christopher Clark, The Sleepwalkers: How Europe Went to War in 1914 (2012) p 481.
Williams, Glen (1989). "6: Canada in the International Political Economy". In Clement, Wallace; Williams, Glen (eds.). The New Canadian Political Economy. Montreal: McGill-Queen's Press - MQUP. pp. 116, 130. ISBN9780773506817. Retrieved . The dependency school, dominant in the 1960s and early 1970s, argued that Canada is an economic colony with a client state. [...] while it might have been possible a decade ago to use a Latin American dependency model when describing Canada, because of its excessive degree of foreign ownership and 'American client state' status, both Canadian capitalists and the Canadian state have now 'come of age.'
Ladwig, Walter C. (2017). The Forgotten Front: Patron-Client Relationships in Counter Insurgency. Cambridge University Press. p. 302. ISBN9781107170773. Retrieved . As with their Cold War counterparts, it was erroneous for American policymakers to believe that the governments of contemporary client states, such as Iraq, Afghanistan, and Pakistan, necessarily shared their desire to defeat radical Islamic insurgents by adhering to the prescriptions of U.S. counterinsurgency doctrine.