Carter Page worked in Russia as an investment banker for Merrill Lynch for 3 years in the 2000s, with some involvement with Russian government-owned natural gas company Gazprom. Source From 2014–2015, Page published a series of articles online praising Russia and Rosneft CEO Igor Sechin, and criticizing American policy. In these articles, Page endorsed the Russian invasion of Ukraine, which he wrote was “precipitated by U.S. meddling,” and compared US support for Ukrainian independence to police killings of black youth. Sources: primary, secondary1, secondary2
From 2010–2015, a Russian spy ring including Igor Sporyshev, Victor Podobnyy (both with Russian intelligence agency SVR), and Evgeny Buryakov (also SVR, posing as a banker) carried out espionage, gathering information on sanctions against Russia and other intelligence useful to Russian businesses. The FBI began investigating this spy ring soon after the Illegals Program concluded, and successfully broke it up in 2015. Source
In 2013, this spy ring attempted to recruit intelligence sources in New York City. As part of this effort, Podobnyy targeted Carter Page, and conversations about Carter Page between Podobnyy and another member of the ring were recorded and were published in the FBI complaint (PDF link) filed against the spy ring in 2015. Podobnyy apparently hinted to Page he could use his Russian connections to “push contracts” with Gazprom in exchange for Page providing him with certain documents. He thought it would succeed because Page was an “idiot” who wanted to earn a lot of money and was seeking to establish some sort of business deal with Gazprom. Page confirmed to the FBI that he provided Podobnyy with documents with information about the energy industry. Source (Page’s identity as “Male 1” in the complaint was first reported by Buzzfeed, and later confirmed by Page.)
Based on this activity, the US government obtained a FISA warrant and had placed Page under surveillance since 2014. Source
On March 21, 2016, Donald Trump revealed that Carter Page was part of his campaign’s foreign policy advisory team, headed by Senator Jeff Sessions. Source (Transcript) This was the first time Carter Page was identified with the Trump campaign.
When contacted about his membership in the Trump campaign, Page indicated he would be advising Trump on energy policy and Russia. Insiders were surprised by the announcement, since Page was an apparent unknown in US and Russian circles familiar with the energy industry. Source
See: Carter Page Travels to Moscow (July 2016)
In September 2016, following allegations about Carter Page meeting with Sechin in Moscow became public, Page announced he was taking a “leave of absence” from the Trump campaign. Source
After Page left the Trump campaign, in October 2016, the FBI obtained a 90-day FISA warrant (since, renewed more than once) to monitor him, based on a belief that he was acting as an agent of the Russian government, and that he knowingly engaged in clandestine intelligence activities on their behalf. Source The warrant request was based on Page’s 2013 conversation with Podobnyy, in addition to other as-yet undisclosed contacts between Page and Russian operatives. Source
With the FISA warrant issued in October, it seems fairly likely that if Page’s December trip to Moscow were related to the subject conspiracy, then the FBI would have obtained key intelligence at this time.