Pan Am and TWA were the kings of intra-European segments until the late 1980's. Pan Am did most of its intra-European flying within Germany in accordance with post-WW II aviation agreements and out of London Heathrow to Frankfurt, Brussels, Amsterdam, and elsewhere.
TWA's intra-Europe flights operated primarily out of the airline's Paris-CDG mini-hub where flights from New York/JFK, St. Louis, Boston, and for a short-time, Washington DC, connected to TWA flights to Geneva, Zurich, and some other cities on and off.
TWA also flew a NYC-FCO-ATH-CAI and reverse route, using a 747-100 to FCO and then a 727 on the FCO-ATH-CAI segment. Late in the 1980's, TWA flew from Brussels to Moscow and also had code-sharing with Malev in 1989 for the Zurich-Budapest route, flown with a Malev 737-200.
TWA also flew JFK-LIS-BCN and this route continued pretty much until 1998, when a JFK-BCN nonstop went year-round with a 757 until it was pulled.
United inherited some intra-Europe routes when it bought Pan Am's London Heathrow operation in 1991 and as such, flew LHR-AMS and LHR-BRU and briefly, the LHR-FRA route. It also for a short time flew out of CDG to ATH, GVA, and ZRH using 727-200's in 1992-3, and also operated to Rome by extending its IAD-MXP to FCO as IAD-MXP-FCO-MXP-IAD.
American very briefly operated ZRH-GVA when it first expanded into Continental Europe in the mid-1980's using a 767-200ER.
Delta has on and off provided intra-Europe flights, first when it bought Pan Am's Transatlantic (non Heathrow routes) to places like Budapest, Bucharest, St. Petersburg, Moscow, Vienna, and Warsaw from Frankfurt that were all spokes to Pan Am's old FRA hub. Now Delta only flies from FRA to BOM having scrapped the intra-Europe network and FRA hub concept entirely.