When he was president, Trump delayed military aid to Ukraine. 

The Washington Post reported:

President Trump told his acting chief of staff, Mick Mulvaney, to hold back almost $400 million in military aid for Ukraine at least a week before a phone call in which Trump is said to have pressured the Ukrainian president to investigate the son of former vice president Joe Biden, according to three senior administration officials.

Zelensky’s “entire” administration was concerned “that the aid that was being cut off to Ukraine by the president was a consequence for their unwillingness, at the time, to investigate the Bidens,” Sen. Chris Murphy (D-Conn.) said, citing his interactions with numerous Ukrainian officials during an early September trip there. Murphy said he heard “directly” from Zelensky about “his concern about why the aid was being cut off to Ukraine,” though the Ukraine’s new president did not specifically broach the subject of a quid pro quo.

The New York Times reported:

President Trump personally ordered his staff to freeze more than $391 million in aid to Ukraine in the days before he pressed the new Ukrainian president to investigate the Democrats’ leading presidential candidate, two senior administration officials said Monday.

The New York Times additionally reported:

Mr. Trump’s order to hold $391 million worth of sniper rifles, rocket-propelled grenades, night vision goggles, medical aid and other equipment the Ukrainian military needed to fight a grinding war against Russian-backed separatists would help pave a path to the president’s impeachment.

The Democratic-led inquiry into Mr. Trump’s dealings with Ukrainethis spring and summer established that the president was actively involved in parallel efforts — both secretive and highly unusual — to bring pressure on a country he viewed with suspicion, if not disdain.

One campaign, spearheaded by Rudolph W. Giuliani, the president’s personal lawyer, aimed to force Ukraine to conduct investigations that could help Mr. Trump politically, including one focused on a potential Democratic 2020 rival, former Vice President Joseph R. Biden Jr.

To justify cutting off aid to Ukraine, Trump slandered Zelensky. The New York Times reported:

The budget office officials had little idea of why Mr. Trump was interested in the topic, but many of the president’s more senior aides were well aware of his feelings about Ukraine. Weeks earlier, in an Oval Office meeting on May 23, with Mr. Sondland, Mr. Mulvaney and Mr. Blair in attendance, Mr. Trump batted away assurances that Mr. Zelensky was committed to confronting corruption.

“They are all corrupt, they are all terrible people,” Mr. Trump said, according to testimony in the impeachment inquiry. 

“This defense relationship, we have gotten some really good benefits from it,” Mr. Esper added, noting that most of the money was being spent on military equipment made in the United States.

Mr. Trump responded that he did not believe Mr. Zelensky’s promises of reform. He emphasized his view that corruption remained endemic and repeated his position that European nations needed to do more for European defense.

“Ukraine is a corrupt country,” the president said. “We are pissing away our money.”

The criticism of Ukrainian leadership came at the same time Trump studiously avoided criticizing Putin for Russia’s military action against Ukraine. Politico reported:

The Trump administration is slow-walking $250 million in military assistance to Ukraine, annoying lawmakers and advocates who argue the funding is critical to keeping Russia at bay.

President Donald Trump asked his national security team to review the funding program, known as the Ukraine Security Assistance Initiative, in order to ensure the money is being used in the best interest of the United States, a senior administration official told POLITICO on Wednesday.

But the delays come amid questions over Trump’s approach to Russia, after a weekend in which the president repeatedly seemed to downplay Moscow’s military intervention in Ukraine and pushed for Russia to be reinstated into the Group of Seven, an annual gathering of the world’s largest advanced economies.

The White House explanation that Trump wants to ensure the money is being spent properly isn’t sitting well with lawmakers on Capitol Hill, where members of both parties have pushed to increase military assistance to Ukraine and U.S. military efforts to deter Russia in Eastern Europe.

NPR reported:

The Trump administration’s hold-up of aid to Ukraine was irregular and likely violated U.S. law, and has far-reaching consequences at home and overseas.

The delay on aid to Ukraine was unusual because it involved military assistance that had bipartisan support.

Trump’s delaying military aid to Ukraine led to his impeachment. CNBC reported:

The Trump administration broke the law by withholding congressionally approved military aid to Ukraine last summer “for a policy reason,” a top government watchdog said Thursday in a scathing report.

The Government Accountability Office’s report came a day after the House of Representatives sent articles of its impeachment of President Donald Trump to the Senate for conduct related to holding back that aid.

Trump refused to release the funds to Ukraine at the same time he was pressuring that country’s new president to announce investigations of former Vice President Joe Biden and of Biden’s son Hunter, who had served on the board of a Ukraine gas company. Joe Biden is the current front-runner for the Democratic presidential nomination.

The funds were only released after the block on the aid became publicly known, sparking the congressional probe which led to the Republican president’s impeachment by the Democratic-controlled House.