Vladimir Putin? You Can't Negotiate with a Thug and a Gangster and a Schoolyard Bully

 Or, 'A bad excuse is good enough for a bully'. 

See 'The Hungry Wolf and the Lamb', Aesop's Fables, 1965 edition).

Alternatively, 'Some people believe that might makes right'. 

Likewise, see 'The Lion's share', ibid.