How is "Rambo" used as a metaphor?
How “Rambo” works as a metaphor depends on the context.
In common speech/slang, to “go Rambo” is to get really violent. You might say that you are going to “go Rambo” on someone if you’re planning to pick a fight with them (and expect to win). It can also mean something more specific, like picking a fight even if the odds are against you.
From the movie series, Rambo has become a metaphor for America, at least in foreign eyes. Other countries see the United States as an essentially American hero, and Rambo himself as summing up the American attitude towards the world. If something bad happens, get overwhelmingly violent in response. Be ultra-macho.
Outside observers have some reason to view America this way. After all, President Reagan said he was inspired by Rambo at times.