One hundred years ago Alfred Land\'e unriddled the Anomalous Zeeman Effect and presaged Electron Spin

In order to commemorate Alfred Land\'e's unriddling of the anomalous Zeeman Effect a century ago, we reconstruct his seminal contribution to atomic physics in light of the atomic models available at the time. Land\'e recognized that the coupling of quantized electronic angular momenta via their vector addition within an atom was the origin of all the apparent mysteries of atomic structure as manifested by the anomalous Zeeman effect. We show to which extent Land\'e's ideas influenced the development of quantum physics, particularly Wolfgang Pauli's path to the exclusion principle. We conclude with Land\'e's brief biography.