Conor, the son of Robert F. Kennedy Jr., has returned home safely and is now encouraging others to get involved.
"Like many people, I was deeply moved by what I saw happening in Ukraine over the past year. I wanted to help. When I heard about Ukraine's International Legion, I knew I was going, and I went to the embassy to enlist the next day," Conor, who dated Taylor for four months in 2012, wrote on Instagram.
Conor, 28, said only one person in the United States knew where he was, as he didn't want friends and family to worry about him. In Ukraine, he only identified himself accurately to one person.
"Going in, I had no prior military experience and wasn't a great shot, but I could carry heavy things and learned fast. I was also willing to die there. So they soon agreed to send me to the northeastern front," he said.
Conor, who is John F. Kennedy's great-nephew, didn't detail how long he was defending Ukraine against Vladimir Putin's Russian forces, but acknowledged it "wasn't long."
"I saw a lot and I felt a lot," he wrote. "I liked being a soldier, more than I had expected. It is scary. But life is simple, and the rewards for finding courage and doing good are substantial."
Despite knowing the risks, the scion said he would do it all over agin.
"This war, like all others, is horrific. The people I met were the bravest I have ever known. My fellow legionnaires — who came from different countries, backgrounds, ideologies — are true freedom fighters," he continued. "As are the citizens I knew, many of whom have lost everything in their long struggle against oligarchy, and toward a democratic system. They know this isn't a war between equals, it's a revolution."
According to Time, Ukrainian President Volodymyr Zelenskyy set up the International Legion in February asking foreigners to help fight Russia. Within a week, 20,000 people from 52 countries signed up, the report said.
The battlefield is a far cry from his day-to-day life, particularly when he was dating the pop star as a teenager. Fans believe that "Begin Again" from Taylor's "Red" album is about him.
"This war will shape the fate of democracy in this century. There's more to say about its politics and the role of western governments there," Conor wrote. "For now, I'll only urge you to help in your personal capacity. Join the legion, help on the border, or send medical supplies. Every day, someone there sacrifices everything for a lasting peace. They can't be asked to act alone."