Even though the PoD is right at the distinction of our forums in before and after 1900 I'm putting this in the after 1900 since all the effects of course will be after. So in OTL, the US under President McKinley joined the Eight Nation Alliance and defeated the Boxer Rebellion in China 1899-1901 being the second time the US joined with European(s) powers in an overseas adventure (the Barbary Wars, 100 years earlier, was in conjunction with the Swedish).
So, the PoD- the US demands a Guantanamo Bay type situation in China, a lease of a military base and surrounding area. Helps trade, sets up an additional point of support for the Philippines, helps contain Japan, an important coaling station, several reasons really make sense.
In fact OTL Secretary of State John Hay asked the American Minister to Japan to see if Japan would object to such a purchasing of a base from the Chinese at Samsah Bay (being in Fukien it was Japanese sphere), the Japanese said that would be unwise and would be the end of the Open Door Policy.
What if the US said "Long hair don't care" to the Japanese and took Samsah Bay as a Guantanamo Bay in East Asia... Japan of course takes it with Hong Kong, Manila, etc during WWII; but let's say the US re-takes it after WWII. The Chinese Civil War continues, and lo and behind look where Samsah Bay is- directly across the Taiwan Straits from Taiwan... can this impact Taiwan at all? Does the presence of an American installation mean that perhaps some or all of Fukien Province remains RoC and not PRC? Certainly in 1948 the PRC is in no position to take an American installation and the US won't let them. The US won't hand it over any time in the 1950s or 1960s because- the US doesn't recognize the PRC at all. During the Korean War once China joins in the fray you have the potential that Samsah Bay is the match stick for WWIII, same during Vietnam War if China makes demands on what can fly out of Samsah Bay and in what direction when going to NV or SV. By the time Nixon opens China in 1971 why give Samsah Bay up? China is still no match and to appease them in such a manner would be opposed by Taiwan, Japan, and South Korea, you'd basically be telling the PRC they have a green light for invasion of Taiwan. Would the US agree to give it up when Britain agrees in 1984 that it will give up all of Hong Kong instead of originally it was only the New Territories and Kowloon that were to be given back after 100 years, since Hong Kong island was given to the UK forever by legal treaty. Or does the US say NO still in 1984 (remember in 1984 China is still backwards and no economic powerhouse) and the UK say "well, shoot, if they say no, we should too and then the US has to support us if China gets beligerent about it". US becomes hypocrites in this case and Thatcher remembers the Falklands War and figures this will play out the same if she stays strong and holds on to Hong Kong.
Once you put a US base on China what happens? I can't see the US giving it up. It's not geopolitically possible after 1945 because of its strategic location. But can it actually change anything?