As part of the federal government 2019 budget, the Pentagon and the State Department set aside a combined $391 million to buy an array of weapons and military training for Ukraine. But the Trump administration sat on the money. With the end of the fiscal year looming, Congress leaned on the White House to free up the cash.

Considering the United States’ strong support for Ukraine since Russia’s 2014 invasion and annexation of the country’s strategic Crimea region, the funding impasse seemed… weird. Then in mid-September, news broke that Trump had been using the money as leverage as he leaned on the Ukrainian government to open an investigation into the son of potential Democratic presidential rival Joe Biden.

Why was that money so important—to Ukraine, and to America? What exactly was in that deal?