It wasn't illegal. He could have met with them all he wanted. Meeting with someone to get dirt on a candidate is as old as the Republic itself.
~Jeordam
https://www.vox.com/world/2017/7/10/15950590/donald-trump-jr-new-york-times-illegal
11 CFR 110.20 - Prohibition on contributions, donations, expenditures, independent expenditures, and disbursements by foreign nationals (52 U.S.C. 30121, 36 U.S.C. 510).
Note there are several other laws he might have violated to, but the campaign finance laws are pretty much clear cut, if its a foreign national you are not allowed to seek a contribution of anything of value.
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You may call this a stupid law, you may say it violates the 1st amendment, and so on, but it is the law.
This is true though, foreign nationals getting involved with US elections has happened ever since the 2nd election with John Adams and may have cost Jefferson the election.
http://www.history.com/news/that-time-a-foreign-government-interfered-in-a-u-s-presidential-election-in-1796
Debating the ethics of what Donald Trump Jr did is very different than debating the legality of what Donald Trump Jr did.
(Also different is debating the morality of what Donald Trump Jr did.)
Note it is not just Donald Trump Jr, but also Paul Manafort (who should know better for he would be familiar with that part of campaign finance law, and Jared Kushner.
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Carry on you two, just pointing out the legal law that is involved since Jeo (wrongly) stated no law that was broken.