Bootstrap dropdown nav menus not working? (and solution)

It’s not immediately obvious unless you’ve run into this before, but the dropdown nav menu feature of Bootstrap requires both jQuery and the bootstrap.js includes. Maybe because I normally don’t use any of the JavaScript features of Bootstrap I haven’t run into this before. It is covered in the install docs here as a requirement, but maybe I’ve not noticed this before. It must be a common issue though, here’s a question on SO.

If you leave out either of these the menu renders as expected but the dropdown doesn’t do anything. Here’s a plunkr with both .js files included as a working example (example HTML from the example here).