When the Department of Justice designated Robert Mueller as special counsel to take over the FBI investigation of the Trump campaign in May 2017, Mueller's initial task was to determine if there had been a conspiracy — an illegal agreement — between the campaign and any Russians to receive anything of value.

When former FBI Director James Comey informed Mueller that he believed Trump fired him because he had declined Trump's order to shut down the investigation of Trump's campaign and of his former national security advisor, retired Lt. Gen. Michael Flynn, Mueller began to investigate whether the president had unlawfully attempted to obstruct those investigations.

We now know why Trump was so anxious for the FBI to leave Flynn alone.

Flynn was charged and pleaded guilty to lying to the FBI about whether he discussed sanctions in a telephone call with then-Russian ambassador to the United States Sergey Kislyak, before Trump became president. Such a communication could have been unlawful if it interfered with American foreign policy.  

So, when Trump learned of the lie, he fired Flynn. Yet in his plea negotiations with Mueller, Flynn revealed why he discussed sanctions with Kislyak — because the pre-presidential Trump asked him to do so. An honest revelation by Trump could have negated Flynn's prosecution. But the revelation never came.

Last week, Attorney General William Barr released publicly a redacted version of Mueller's final report. That report concluded that notwithstanding 127 confirmed communications between the campaign and Russians from July 2015 to November 2016 (Trump said there were none), the government could not prove the existence of a conspiracy.

On obstruction, the report concluded that notwithstanding numerous obstructive events engaged in by the president personally, the special counsel would not charge the president and would leave the resolution of obstruction of justice to Congress. Congress, of course, cannot bring criminal charges, but it can impeach.

Trump initially claimed that he had been completely exonerated by Mueller — even though the word "exoneration" and the concept of DOJ exoneration are alien to our legal system. Then, after he learned of the dozen or so documented events of obstruction described in the report, Trump used a barnyard epithet to describe it.

The Constitution prescribes treason, bribery or other high crimes and misdemeanors as the sole bases for impeachment. We know that obstruction of justice constitutes an impeachable offence under the "high crimes and misdemeanors" rubric because both presidents in the modern era who were subject to impeachment proceedings — Richard Nixon and Bill Clinton — were charged with obstructing justice.