Ukraine to Syria to his government's campaign to upend the last US presidential election.
Around 2,500 people gathered in Helsinki for a peaceful demonstration to promote human rights, the protection of the environment and democracy ahead of the summit on Sunday.
"Now, you wouldn't think of the European Union but they're a foe".
He also says that Russian Federation is a foe "in certain respects" and that China is a foe "economically". China is a foe economically, certainly they are a foe. "It means that they are competitive". But Trump has consistently resisted the suggestion that he confront Putin over the matter, saying Putin has repeatedly denied involvement.
A billboard is pictured along President Donald Trump's route to the summit in Helsinki with Russia's President Vladimir Putin.
One poster, with a headline from 2004, reads in Russian: "Russian reporter who criticized Putin gains asylum in Britain".
One protester held a sign "Build saunas not war!"
But in Helsinki, which lies close to the Russian border, there was plenty of heat on Putin too.
And the two men are nationalists who promised to make their countries "great again" - Putin after the instability that followed the collapse of the USSR and Trump after what he saw as the decline of American industry. Putin was expected to attend the soccer World Cup final between France and Croatia at the Luzhniki stadium in Moscow on Sunday.
Trump later attempted to disavow his words, calling the interview "fake news", after it proved embarrassing during his meeting with May.
The president also told CBS News in an interview that he "hadn't thought" about asking Putin to extradite the dozen Russian military intelligence officers indicted this past week in Washington on charges related to the hacking of Democratic targets in the 2016 USA election.
This is a common refrain from Trump, who has called for better relations with Russian Federation since he was a candidate.
In the CBS interview, Trump appeared to lay blame for the hacking on the Obama administration and the Democratic Party, not Russian Federation. "I think it's a good thing to meet", he said. "I would say, in fact, it strengthens his hand". So having meetings with Russia, China, North Korea, I believe in it. "Trump will be responsive to that tack", she said.
"We've got a fraught bilateral relationship. Why didn't they do something about it, especially when it was reported that President Obama was informed by the Federal Bureau of Investigation in September, before the Election?" he asked.
"Putin has proven himself to be incredibly savvy at reading personalities and characters", said Alina Polyakova, a foreign policy fellow at the Brookings Institution in Washington, noting that Putin was trained as a KGB spy.
Maas welcomed the meeting, at which the United States president has said he will raise the issue of nuclear weapons reduction.
The outlines of what would be a hugely controversial deal have emerged in press reports that suggest Putin is keen to exploit Trump's desire to pull United States troops out of Syria by offering to rein in Iranian influence there. In exchange for Russia's kicking Iran out of Syria, Trump could recognize Russia's annexation of Crimea or draw down USA sanctions.
Putin has less reason to cheer Trump's imposition of trade tariffs on countries including Russian Federation, and from his decision to abandon a landmark nuclear pact with Iran.
While Trump speaks off the cuff and often angrily contradicts his own advisors - or himself - Putin is never caught off-guard in public and rarely raises more than an eyebrow to express his emotions.