STL was very much a large hub for AA after it took over from TWA and flew to LGW and CDG. STL was never a big op for European flights, basically to the mainstays of London and Paris. JFK for TWA was always the European hub. AA continued this for a couple years past the buy out. The hub was well over 400 daily flights and AA continued the London and Paris flights on their metal. The hub was paired back due to the industry issues that followed 9/11 in the fall of 2003, where the schedule was halved to 200 daily flights and the flights to Europe and Hawaii were halted. The only “long haul” flight that remained was weekend only service to SJU. The new mini hub at STL actually performed fairly well enough for AA that some cities were added back to the hub and some cities that had service downgraded to regional service saw mainline come back and some that had prop service was upgraded to regional jet service. There had been some talk of AA even reinstating the LGW route at one point maybe around 2005/2006 or so but alas another recession kicked in around 2008 and the hub was finally dismantled around late 2009 to 2010
AA bought TWA to knock out a competitor, something it had done a few times over its post-deregulation history (Air Cal, Reno Air). AA argued it wanted the STL hub to relieve pressure on ORD, which is ironic, considering how much ORD has been scaled back by AA since, to essentially a domestic hub with token service to LHR year round and some seasonal services (ORD was once AA's primary gateway to Europe). It was one of the most ill-timed airline "mergers" ever but TWA was limping along and running on less than fumes at the time. Had TWA remained a stand alone airline past-9/11, it would have shut down within weeks.