I am looking at flights to Bangkok from Washington D.C. Most flights go via Iceland, Tokyo and Seoul. Will I need to go through customs at these airports? The layovers are only 2 and a 1/2 hours, which might cut it close if i must go through their customs prior to Bangkok customs.I appriciate any feedback!
For most International-to-International connections you are not required to go through customs unless you exit the airport or in some cases need to change terminals. Not all terminal changes require going through customs but some do.
In Tokyo and Iceland, you can do the transit without going through customs. However, in Iceland, you will go through security again to be sure that you comply with Shengen rules when arriving in Iceland. It does not usually take so long, so with 2 hours, you should be fine as long as there is no delay for the incoming flight.
No idea about Seoul. Never been there or transited through it, so perhaps someone else will have the answer for there.