Smoking indoors became illegal through a gradual process with different key milestones:

• In the US federal government, smoking indoors in federal buildings was banned by President Bill Clinton's Executive Order 13058, signed in August 1997. This banned smoking in all federal executive branch buildings and significantly protected workers and visitors from exposure to secondhand smoke.

• At the state level, California was the first US state to ban smoking in nearly every indoor workplace, including public buildings and restaurants, with their law starting in 1995. Later extensions included bars and gaming clubs in 1998. California's laws were pioneering and influential in indoor smoking restrictions across the US.

• Other states followed, with Delaware implementing a comprehensive smoke-free law in 2002, New York in 2003, Massachusetts in 2004, and many more after.

• On a broader scale, England made it illegal to smoke in enclosed public places and workplaces starting July 1, 2007, including work vehicles and public transport.

In summary, the federal ban on smoking indoors in US government buildings started in 1997, California led the state-level bans in the mid-1990s, and many jurisdictions expanded bans through the 2000s and beyond. England and other countries implemented smoking bans indoors mostly in the 2000s as well. The timeline varies by country and region. The landmark federal US ban of 1997 is a significant milestone for indoor smoking legality.