I think Amsterdam Schipol has been overlooked. Think about how small the country that it's in is. This is just my guess, but I don't think many tourists go to Holland in comparison with the U.K., France, or Germany. But, there is plenty of nonstop service EHAM from the USA and Canada. After arriving from Northwest, KLM, or another airline, you can take KLM to anywhere in Europe, and many places in the Middle East, Asia, and Africa. My aunt and uncle always do this when they go to Nairobi. Drive from here (around KPHL), KEWR or KJFK on NWA/KLM to AMS, and then on to Nairobi. They get tons of miles too, and the NWA/KLM alliance doesn't even seem to care which you use to cross the ocean anymore.
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