Did President Donald Trump withhold military aid from ally Ukraine for his own political reasons? That is the big question at the heart of the Democrat-led impeachment inquiry.
Ukraine is not in Nato and it has struggled with years of corruption. But its future direction is of geopolitical importance.
Why does Ukraine matter to US?
For much of its 28-year independent history it has been unclear whether Ukraine would end up aligned with Russia or the West.
In 2014, that uncertainty appeared to come to an end. After a bloody street revolution, Ukraine's new leaders asserted that the country's future lay with closer association with Europe and the West. Russia was now seen as the enemy and it responded by seizing Crimea and supporting an armed uprising in eastern Ukraine that has so far cost more than 13,000 lives.
Under the US Obama administration, Ukrainians' right to decide their own future, and to resist Russian aggression, was defended as an important ideological principle.
That changed when President Trump took office, and Ukraine's government has not been able to rely on America's active support in the same way since.
How much military aid does the US give?
The US has a longstanding relationship with the Ukrainian military and has committed about $1.5bn (£1.2bn; €1.4bn) in aid since 2014.
Much of that has been spent training soldiers and in efforts to modernise the dated way the Ukrainian army is organised and operates.
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The latest tranche of assistance, which was held up and then released by the Trump administration, is worth $391m and includes a range of weapons and technical assistance.
How important is US military aid and support to Ukraine?
It is important on both a military and symbolic level.
Where once the Ukrainian military knew that the US had their backs, this can no longer be taken for granted.
The wavering over assistance, and President Trump's frequent derogatory words about Ukraine, will almost certainly have strengthened Moscow's hand in peace negotiations.
How is Ukraine linked to Trump impeachment inquiry?
The impeachment inquiry touches on the 2016 US presidential election campaign and also President Trump's possible opponent in 2020 - Joe Biden. Ukraine is involved in both.
So far no Ukrainian has been called to testify. The inquiry is instead focusing on the official and "irregular" US channels of communication between Washington and Kyiv.
Why does the 2016 election keep coming up?
President Trump's lukewarm attitude towards Ukraine dates back to the election campaign of 2016.
At the time, Mr Trump's campaign manager - Paul Manafort - was forced to resign after documents emerged in Ukraine that suggested he had received off-the-books payments from a pro-Russian political party.
President Trump and his supporters say that by releasing the documents, Ukrainian officials unfairly interfered to support his Democrat opponent Hillary Clinton.
Despite the best efforts of President Trump's lawyer Rudy Giuliani, the evidence to support the claims remains extremely thin. Meanwhile, Mr Manafort has been jailed for financial crimes.
What about the Bidens?
This has turned into the central part of the impeachment inquiry with both parties approaching from a different angle.
It concerns a period in 2014-15 when Vice-President Biden was the Obama administration's point-man on Ukraine. At the same time his son, Hunter, had a lucrative directorship with a Ukrainian gas company.
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