There's a great deal of needless confusion concerning what prompted the investigation concerning alleged Trump-Russian-government collusion. That's because the term investigation is used ambiguously. There was/is more than one investigation. There's also the issue of what it took to get the American people (enough of them) on board to launch further and even new investigations, which is the most important aspect.
To be clear, the Steele Dossier and the alleged Russian-government hacks of the DNC and Podesta were the main aspects used to sell the public, which allowed for further and new investigations, including in the US legislature.
The exact thing and moment that triggered everything was a decision or suggestion by someone to even begin to push a narrative as a false-propaganda tactic. When the leaders of the CIA, NSA, FBI, etc., exactly fell in line on that, when it was agreed as a group, and when each took which individual steps to roll it out to cause all of the various investigations would be interesting and useful to know.
Where I believe it likely started though was in Hillary Clinton's head. It all reminds me of the "vast right-wing conspiracy" she said existed to get her husband, Bill Clinton, President at the time, via Bill's and Monica Lewinsky's sexual relations. It all looks to me like Hillary Clinton simply took the page from the standard political play book, which includes all the known dirty tricks down through the centuries, and used it in reverse, against Trump, only in this case, unlike Bill Clinton, Trump didn't do the thing alleged against him as far as any supposed evidence shows.
As for House Intelligence Committee Chairman Devin Nunes, he's fighting with one arm because he's not willing to use un-neocon evidence to aid in defeating the Clinton-wing of the Democratic Party and those in the Republican Party (such as the Bushes) who more than go along with her on hating Trump. He hasn't yet challenged the allegations against Russia concerning hacking, all of which hacking by Russia remains pure, even wild, speculation at best, as all of the supposed evidence presented is easily created to make things appear to be Russian-government actions. Those making the accusations have not been put under oath, forced to undergo cross-examination, and to produce all of their evidence to substantiate their allegations.
We are talking about the most important issues facing the planet, as we are talking about the real potential for WWIII between the US and allies against Russia and allies. If ever there were an instance where excruciatingly exact and thorough due process of law should be required and applied, this is it. Exactly who would be deprived of life or liberty or property if some lower standard were used allowing the gravest error to prevail resulting in total nuclear war?
People who say we shouldn't apply such a high standard in matters of foreign policy are dangerous and ought not be in charge of anything involving governmental policies and practices concerning the safety and security of others.