If you are using react-router v4 you need to install react-router-dom as well. After that, import BrowserRouter from react-router-dom and switch Router for BrowserRouter. It seems that v4 change several things. Also, the react-router documentation is outdated. This is my working code:
import React from 'react';
import ReactDOM from 'react-dom';
import { BrowserRouter, Route } from 'react-router-dom'
import App from './components/App';
ReactDOM.render((
<BrowserRouter>
<Route path="https://stackoverflow.com/" component={App}/>
</BrowserRouter>
),
document.getElementById('root')
);
Source