In 1984 Congress passed the law that made it illegal for anyone in the United States under the age of 21 to purchase or publicly possess alcohol. While drinking laws are and always have been a states issue, the federal government was able to enforce the minimum age by making it a part of the Federal Aid Highway Act .
So for 28 years, states have been compelled to keep the minimum legal drinking age at 21 or face losing their federal highway funding.
Didn’t the Roberts opinion on Obamascare just say they couldn’t do that?
“So for 28 years, states have been compelled to keep the minimum legal drinking age at 21 or face losing their federal highway funding.”
This is also how the 55mph speed limit was enforced.
The Feds do this in lots of areas - a big one that comes to mind is how federal public school funding K through graduate schools have to jump through Fed hoops to get meals, loans, infrastructure, grants, etc. funding.
See also CDBG - Community Development Block Grants, Enterprise zones, etc.
More examples?