Michael Sheen's relationship with Sarah Silverman ended because of Brexit and Donald Trump.
The Masters of Sex actor split from the 47-year-old comic in February after four years together and he admitted the fact they both wanted to be more politically involved in their home countries following the UK referendum result and the election of the controversial Apprentice star contributed to the split.
He said: "We both had very similar drives, and yet to act on those drives pulled us in different directions - because she is American and I'm Welsh.
"After the Brexit vote, and the election where Trump became president, we both felt in different ways we wanted to get more involved.
"That led to her doing her show I Love You America, and it led to me wanting to address the issues that I thought led some people to vote the way they did about Brexit, in the area I come from and others like it."
While the pair have remained friends, they don't regret sacrificing their relationship for the sake of their beliefs.
Sheen - who has been working with small grassroots groups in his native Wales - told the Daily Telegraph newspaper: "I felt a responsibility to do something, but it did mean coming back here - which was difficult for us, because we were very important to each other. But we both acknowledge that each of us had to do what we needed to do."
The 49-year-old actor - who has daughter Lily, 19, with former partner Kate Beckinsale - also admitted his acting career has taken a back seat while he focuses on his activism.
He said: "I have two jobs now, essentially. Acting takes second place."
Sheen is also considering returning to the theatre, and has been considering a show about "political historical socio-economic stuff, a one-man show with very low production values".