A ceasefire deal reached last year in Belarus -- the so-called Minsk agreement -- is a distant memory in eastern Ukraine, where CNN witnessed intense fighting in the last several days.
Unrest there has been on the rise, and Russia has recently conducted military drills in Crimea, the peninsula it annexed from Ukraine in 2014 -- a move most nations do not recognize as legitimate.
Had you asked him in 2013, Poroshenko said, if it would have been possible for Russia to "occupy the Crimea," he would have said "no, this is not possible -- there is some red line, and Putin [will] not cross this line."
"If you asked me in January, year 2014," he went on, if it was possible that "thousands of Russian regular troops will penetrate on Ukrainian territory in the east of my country in July and August," I would have said, "no, this is not possible."
With those moves, he said, the world "is completely changed." "Russian aggression completely destroyed the post-war global security system," he said.
Twenty-five years after independence, he said, the fight goes on
-- "fighting for freedom, fighting for democracy, fighting for sovereignty and territorial integrity."
Ukraine has become a somewhat surprisingly prominent issue in the American election. That's due in part to Donald Trump's former campaign chairman, Paul Manafort,
who has served as an adviser to former Ukrainian President Viktor Yanukovych, who fled office in 2014 after massive protests in the country.
It was recently announced that Ukraine's anti-corruption bureau is investigating Manafort and whether he received millions in potentially illegal cash payments from the country's former ruling party.
Poroshenko would not comment on the specifics of the case, but pledged that any possible charges -- against Manafort or anyone else -- would only come after a "transparent and independent investigation."