The way I see it is that you have 2 problems to solve here. The first is how to set the innerHTML of an element in React. The other is how to get a specific HTML to render depending on a given variable (e.g the current route, the input of a textfield, etc).

1. Setting the innerHTML of an element

You can do this with the dangerouslySetInnerHTML prop. As the name suggests it sets the innerHTML of the said element to whatever you specify... and yes, the "dangerously" is accurate as it's intended to make you think twice before using this feature.

The Official Documentation reads as follows:

Improper use of the innerHTML can open you up to a cross-site scripting (XSS) attack. Sanitizing user input for display is notoriously error-prone, and failure to properly sanitize is one of the leading causes of web vulnerabilities on the internet.

Check out this Demo or the snippet below.

var Demo = React.createClass({

  getInitialState: function() {
    return {showExternalHTML: false};
  },
  
  render: function() {
    return (
      <div>
        <button onClick={this.toggleExternalHTML}>Toggle Html</button>
        {this.state.showExternalHTML ? <div>
          <div dangerouslySetInnerHTML={this.createMarkup()} ></div>
        </div> : null}
      </div>
    );
  },
  
  toggleExternalHTML: function() {
    this.setState({showExternalHTML: !this.state.showExternalHTML});
  },
  
  createMarkup: function() { 
    return {__html: '<div class="ext">Hello!</div>'};
  }

});

ReactDOM.render(
  <Demo />,
  document.getElementById('container')
);

.ext {
  margin-top: 20px;
  width: 100%;
  height: 100px;
  background: green;
  color: white;
  font-size: 40px;
  text-align: center;
  line-height: 100px;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="container"></div>

2. Fetching the HTML from an external source

Note that the above example does not actually get the HTML from an external file, but is entered directly as a string.

One simple way to do dynamically fetch a choose a specific file would be to let your backend (e.g php) read the file from a local folder, parse the text, and send it back through an AJAX request.

Example

//Your React component
fetchExternalHTML: function(fileName) {
  Ajax.getJSON('/myAPI/getExternalHTML/' + fileName).then(
    response => {
      this.setState({
        extHTML: response
      });
    }, err => {
      //handle your error here
    }
  );
}

If your template.html file is just HTML and not a React component, then you can't require it in the same way you would do with a JS file.

However, if you are using Browserify — there is a transform called stringify which will allow you to require non-js files as strings. Once you have added the transform, you will be able to require HTML files and they will export as though they were just strings.

Once you have required the HTML file, you'll have to inject the HTML string into your component, using the dangerouslySetInnerHTML prop.

var __html = require('./template.html');
var template = { __html: __html };

React.module.exports = React.createClass({
  render: function() {
    return(
      <div dangerouslySetInnerHTML={template} />
    );
  }
});

This goes against a lot of what React is about though. It would be more natural to create your templates as React components with JSX, rather than as regular HTML files.

The JSX syntax makes it trivially easy to express structured data, like HTML, especially when you use stateless function components.

If your template.html file looked something like this

<div class='foo'>
  <h1>Hello</h1>
  <p>Some paragraph text</p>
  <button>Click</button>
</div>

Then you could convert it instead to a JSX file that looked like this.

module.exports = function(props) {
  return (
    <div className='foo'>
      <h1>Hello</h1>
      <p>Some paragraph text</p>
      <button>Click</button>
    </div>
  );
};

Then you can require and use it without needing stringify.

var Template = require('./template');

module.exports = React.createClass({
  render: function() {
    var bar = 'baz';
    return(
      <Template foo={bar}/>
    );
  }
});

It maintains all of the structure of the original file, but leverages the flexibility of React's props model and allows for compile time syntax checking, unlike a regular HTML file.

Hello React

You can import or require the html file and use it in the src attribute,

var perf =require('./template.html');

class Index extends React.Component {
   render(){
      return (
         <iframe src={perf }></iframe>   /* like this */
      );
   }
}
export default Index;

You can use dangerouslySetInnerHTML to insert html in react component. This will render the whole html inside the div tag.

const content = `<html>
<head>
    <meta content="text/html; charset=UTF-8" http-equiv="content-type">
    <style type="text/css">
       @import url('https://themes.googleusercontent.com/fonts/css?kit=fpjTOVmNbO4Lz34iLyptLUXza5VhXqVC6o75Eld_V98');.lst-kix_list_1-3>li:before{content:"\0025cf  "}.lst-kix_l.......
    </style>
</head>
<body class="c16">
    <div>Test</div>
</body>
</html>`


class TermsAndConditionsPage extends React.Component {
    render() {
       return <div dangerouslySetInnerHTML={{ __html: content }}/>
    }
}

From the docs:

dangerouslySetInnerHTML is React’s replacement for using innerHTML in the browser DOM. In general, setting HTML from code is risky because it’s easy to inadvertently expose your users to a cross-site scripting > (XSS) attack. So, you can set HTML directly from React, but you have to type out dangerouslySetInnerHTML and pass an object with a __html key, to remind yourself that it’s dangerous.