President Donald Trump says there’s nothing wrong with the phone call where he asked Ukrainian President Volodymyr Zelensky to investigate former Vice President and 2020 Democratic presidential candidate Joe Biden as well as his son Hunter Biden. But it’s against US law for a candidate to solicit things of value from foreign entities and House Democrats have opened an impeachment investigation over it.

Here’s a look at the key events in the story, going back to 2014. This is pulled mostly from CNN’s body of fact checks on this topic, but also includes background published in the whistleblower complaint, The New York Times, The Washington Post, The New Yorker and from an extremely detailed timeline curated on the website Just Security.