Add Bootstrap dropdown class to a nav menu

How to build this menu in WordPress

<div class="collapse navbar-collapse" id="collapse-1">
                <ul class="nav navbar-nav">
                    <li><a href="#">our story</a></li>
                    <li><a href="#">our vision</a></li>
                    <li class="dropdown">
                        <a href="#">History</a>
                        <ul class="dropdown-menu">
                            <li><a href="#">History 1</a></li>
                            <li><a href="#">History 2</a></li>
                            <li><a href="#">History 3</a></li>

                        </ul>
                    </li>

                </ul>
            </div>

WordPress Code:

<div class="collapse navbar-collapse" id="collapse-1">

<?php
wp_nav_menu( array(
    'theme_location' => 'header',
    'menu_class' => 'nav navbar-nav',
    'fallback_cb' => false
) );
?>

</div>

The easiest way to do this is to use an off-the-shelf solution. There is a WP_Bootstrap_Navwalker class which extends WordPress’ native Walker_Nav_Menu class and makes your Navigation Menus ready for Bootstrap 3 or 4. Download it from GitHub.

Add it to your theme, then add the following to the functions.php:

<?php
require_once('path-to-the-directory/wp-bootstrap-navwalker.php');

Change path-to-the-directory/ to fit your needs.

Next, alter your wp_nav_menu() with the following code:

<?php
wp_nav_menu( array(
    'menu'              => 'header', // match name to yours
    'theme_location'    => 'header',
    'container'         => 'div', // no need to wrap `wp_nav_menu` manually
    'container_class'   => 'collapse navbar-collapse',
    'container_id'      => 'collapse-1',
    'menu_class'        => 'nav navbar-nav',
    'fallback_cb'       => false,
    'walker'            => new WP_Bootstrap_Navwalker() // Use different Walker
));

Note, that you don’t need the <div class="collapse navbar-collapse" id="collapse-1"> anymore as it will be added by wp_nav_menu() with proper CSS classes and id.

Also, read the WP_Bootstrap_Navwalker README.md file carefully.