The origins of the impeachment inquiry engulfing President Trump and his administration go back to the election this spring of a new Ukrainian president, Volodymyr Zelensky. Over the next five months, White House emissaries pressured Mr. Zelensky to announce that Ukraine would investigate two matters: Burisma Holdings, a Ukrainian gas company that hired Hunter Biden, the younger son of former Vice President Joseph R. Biden Jr., and whether Ukraine had meddled in the 2016 American presidential election.
The demands alarmed numerous officials, including an intelligence officer who filed a whistle-blower complaint that eventually reached Congress and set off the impeachment inquiry.
APRIL 21
Volodymyr Zelensky is elected president of Ukraine. Soon after, allies of President Trump’s personal lawyer Rudolph W. Giuliani relay a message that Mr. Giuliani wants to meet with him. Mr. Zelensky puts him off, noting that he has not yet taken office.
May 6
The State Department announces that Marie L. Yovanovitch, the American ambassador to Ukraine, has been recalled to Washington months ahead of schedule after Mr. Giuliani and his allies spread unfounded allegations that she was disloyal to Mr. Trump. About the same time, John R. Bolton, the White House national security adviser, warns a deputy: “Giuliani’s a hand grenade who’s going to blow everybody up.”
MAY 10
Mr. Giuliani tells Fox News that Mr. Zelensky seems surrounded by operatives allied with the Democratic Party and “enemies of the president, and, in some cases, enemies of the United States.” A few days later, an associate of Mr. Giuliani’s warns the Ukrainians that American aid could be withheld, according to the associate’s lawyer, although others dispute that timing.
MAY 23
In an Oval Office meeting with Mr. Trump, Kurt D. Volker, then the special envoy to Ukraine; Gordon D. Sondland, the United States ambassador to the European Union; and Rick Perry, the energy secretary, all describe Mr. Zelensky as a committed reformer who deserves American support. All three attended Mr. Zelensky’s swearing-in in Kiev three days earlier.
Mr. Trump replies that Ukraine is full of “terrible” and corrupt people who had “tried to take me down.” He orders the officials to coordinate future Ukraine matters with Mr. Giuliani.
Later, Mr. Giuliani tells Mr. Sondland that the Ukrainians should publicly announce investigations into Burisma and the 2016 election.
JULY 10
At the Trump International Hotel in Washington, Andriy Yermak, a top adviser to Mr. Zelensky, asks Mr. Volker to connect him to Mr. Giuliani. The two men later meet in Madrid.
At a White House meeting later that day in Mr. Bolton’s office, two Ukrainian officials press for an Oval Office meeting between Mr. Trump and Mr. Zelensky. Mr. Sondland blurts out that Mick Mulvaney, the acting White House chief of staff, promised that Mr. Zelensky would be invited if Ukraine announces “investigations.”
Mr. Bolton halts the meeting. At a follow-up meeting, Mr. Sondland again presses the Ukrainians to announce investigations, this time specifying Burisma and the 2016 election as targets. Fiona Hill, one of Mr. Bolton’s top deputies, calls that session to a halt.
She and Lt. Col. Alexander S. Vindman, her subordinate, report the meetings to John A. Eisenberg, the chief legal adviser to the National Security Council. Mr. Bolton tells Ms. Hill to deliver a message from him: “I am not part of whatever drug deal Sondland and Mulvaney are cooking up.”
JULY 18
In a secure conference call with national security officials, an Office of Management and Budget staff member announces that Mr. Trump was freezing $391 million in security aid to Ukraine until further notice, citing a presidential order to the budget office.
JULY 25
During a phone call with Mr. Zelensky, Mr. Trump raises the favors he is seeking, including a Biden-related inquiry. Afterward, Mr. Vindman, one of the note-takers, reports the call to Mr. Eisenberg.
AUG. 9
The Ukrainians have learned by this date that military aid is frozen and they should speak with Mr. Mulvaney.
“I think POTUS really wants the deliverable,” Mr. Sondland texts Mr. Volker.
Mr. Sondland and Mr. Volker are working on draft language for Mr. Zelensky’s announcement of the investigations. Mr. Yermak says the White House should nail down Mr. Zelensky’s Oval Office invite first.
AUG. 12
A C.I.A. officer detailed to the White House files a whistle-blower complaint about the July 25 call and other concerns. It begins a slow trek to Congress.
AUG. 16
In a memo prepared for Mr. Bolton, Colonel Vindman states that the National Security Council, the State Department and the Defense Department all agree that military assistance to Ukraine should be released. But in a meeting, Mr. Trump rejects the recommendation. An aide later says the “president doesn’t want to provide any assistance at all.”
SEPT. 1
In Warsaw, Mr. Zelensky asks Vice President Mike Pence about the military aid, but Mr. Pence says only that he will speak to Mr. Trump. Mr. Sondland tells Mr. Yermak not to expect the money unless Mr. Zelensky publicly announces a Burisma investigation.
Mr. Zelensky later prepares an announcement for a Sept. 13 CNN appearance, which the Ukrainians hope will satisfy Mr. Trump.
SEPT. 7
Mr. Sondland calls Mr. Trump, asking specifically what he wants from Ukraine. Mr. Trump replies “nothing” and insists that there is “no quid pro quo, ” Mr. Sondland later says.
SEPT. 11
Two days after a House committee is notified of the whistle-blower complaint and opens an investigation, the White House reverses course and releases its hold on the military aid. Plans for Mr. Zelensky’s CNN interview are scuttled.
SEPT. 25
One day after Speaker Nancy Pelosi announces a formal impeachment inquiry, the White House releases a transcript of Mr. Trump’s July 25 call with Mr. Zelensky. Mr. Trump and Mr. Zelensky hold a joint news conference at the United Nations during the General Assembly session. The whistle-blower complaint is delivered to Congress.
Nov. 13
The impeachment inquiry is scheduled to begin public hearings.