Actually, UA started operating JFK-NRT in 1986 after it acquired the Pacific routes of Pan Am. San Francisco and Los Angeles, as well as Seattle/Tacoma were all served on a nonstop basis from JFK. Other than that, United had a huge terminal and very few flights. DC10-10's, B767-200's, and the 747-200 were the UA aircraft seen there, from what I remember.
United started flying to Europe in 1990 with flights from ORD and IAD to Paris and Frankfurt. In 1991, UA acquired Pan Am's Heathrow routes.
In 1997, United switched from a 747-200 to a 747-400 to operate the JFK to NRT flight. The 747-200 had trouble making the route nonstop on a regular basis because of head winds. I flew the 747-200 to NRT in November 1996 and we had to make a refueling stop in Sapporo. Two weeks later, the same thing happened to a colleague.
United has since bifurcated its ops at JFK, with LAX and SFO operating from T6,
the old and decrepit TWA domestic terminal. LHR, EZE, GRU, SEA, SJU, BOS, IAD, NRT, and soon, HKG (CLK) are served from the BA terminal, #7, which is also being remodeled.
United has never been a major operator at JFK. However, it does succeed with its O&D strategy to the cities it serves.