There have been impeachment trials in the United States before, but never in an environment as polarised as this one. Following the Mueller investigations, it looked like the current era of political tribalism would prevent House Speaker Nancy Pelosi from even bringing forward articles of impeachment against US President Donald Trump. Why imperil Democratic candidates in swing states for an inherently political process that would just die in the Republican-controlled Senate? The political maths didn't add up.

But the Ukraine revelations changed that calculus. The speed that the Ukraine developments hit, and the severity of the charges - a sitting US president withholding taxpayer money from an ally unless they opened an investigation into his political opponent and their family - provided the political punch that the Mueller investigations never did (or arguably ever could).