JSFiddle - React, Tailwind, and code Playground

HTML

<nav>
  <a href="#">Menu Item</a>
  <a href="#">A Longer Menu Item</a>
  <a href="#">A Very Very Long Menu Item</a>
  <a href="#">Menu Item</a>
</nav>

CSS

nav {
  display: flex;
}

a {
  flex: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  text-align: center;
  padding: 24px;
  background: darkblue;
  color: #fff;
  text-decoration: none;
}

a + a {
  border-left: 2px solid gray;
}

@media (max-width: 600px) {
  nav {
    flex-wrap: wrap;
  }

  a {
    flex-basis: 100%;
  }

  a + a {
    border-left: none;
    border-top: 2px solid gray;
  }

}