= Maxwell's equations are Lorentz invariant
{c}
<Subtle is the Lord by Abraham Pais (1982)> chapter III "Relativity, the special theory" mentions that this fact and its importance (we want the laws of physics to look the same on all <inertial frames>, AKA <Lorentz covariance>) was first fully relized by <poincaré> in 1905.
And at that same time <poincaré> also immediately started to think about the other <fundamental force> then known: <gravity>, and off the bat realized that <gravitational waves> must exist. <general relativities> is probably just "the simplest way to make gravity <Lorentz covariant>".
Bibliography:
* https://physics.stackexchange.com/questions/219474/proof-that-maxwell-equations-are-lorentz-invariant