Was it treason?

According to the US constitution, Mr Trump Jr’s actions could not be classified as treasonous.

Article 3, Section 3 of the Constitution lays out the definition of treason used in US criminal prosecutions: "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort."

While the US and Russia often have a strained relationship, they are by no means at war.

What other laws could have been broken?

Instead, experts have suggested Mr Trump Jr may have breached campaign finance law, which prohibits “contributions and donations by foreign nationals” to election candidates and bars a person from soliciting, accepting, or receiving a contribution or donation from a foreign national.

“With every attempt at transparency Donald Trump Jr. digs himself more deeply into the hole of criminality,” Michael Gerhardt, who teaches at the University of North Carolina School of Law in Chapel Hill, told Slate. "He appears to have gone into that meeting—and likely others—looking for something of value—dirt on Mrs Clinton—from sources he should have stayed away from. His judgment was bad, to say the least."

The law typically applies to monetary campaign contributions, but courts might consider information such as opposition research to be something of value.