I highly encourage you to read this article
It is from last year, but this is an important to understand the series of events that led to these Russian relationships.
Trump's dad to Roy Cohn to Paul Manafort to Russia. Setting up dictators is what Manafort and his partners have been doing for decades. It is all connected.
His work necessarily entails secrecy. Although his client list has included chunks of the Fortune 500, he has also built a booming business working with dictators. As Roger Stone has boasted about their now-disbanded firm: Black, Manafort, Stone, and Kelly, lined up most of the dictators of the world we could find.
Dictators are in the eye of the beholder. Manafort had a special gift for changing how dictators are beheld by American eyes. He would recast them as noble heroesvenerated by Washington think tanks, deluged with money from Congress.
And it also provides detail on why John McCain may be taking this so personally...
http://www.slate.com/articles/news_and_politics/politics/2016/04/paul_manafort_isn_t_a_gop_retread_he_s_made_a_career_of_reinventing_tyrants.html