1979: Cigars and Pipes banned on aircraft
1988: Smoking banned on US Domestic flights under 2 hours
1990: Smoking banned on US Domestic flights under 6 hours
1998: Smoking banned on all US Domestic flights
2000: Federal law introduced banning smoking on all flights by US airlines.
I can't seem to find any information about when it was banned in the UK, other than the mid 1990s.
There is no overall law or treaty that bans smoking on all international flights. See related links for information about which countries (most of them) have banned smoking on aircraft departing from their airports.
The Framework Convention on Tobacco Control (FCTC) is the world's first public health treaty. The FCTC entered into force on 27 February 2005. The FCTC contains different measures designed to reduce the devastating impact of tobacco on health and economics. As of January 2009, 168 countries have signed the treaty, and 161 countries have ratified the treaty, becoming Parties to the FCTC.