B6 flies international, CUN, SXM, AUA, BDA, and many other places I probaly missed. Just to provide an example. Your idea of a "domestic" (not domestic-only) airline not joining an international alliance is false, AS codeshares with many airlines like AA/DL/BA/Any airline into SEA not US/UA. AS would be part of an alliance but achieves the same result with multitudinous airline partners (alliance of AS allied airlines).