It was in the late 19th century when humanity started to learn about the atom. Naturally as humans their first response was not "how can we generate loads of free energy for all" but "let's make the biggest baddest bomb ever". However such a massive force comes with a risk of massive unintended destruction. Strangely for humans they actual decided to test their bombs. They put massive, fall out causing bombs on the surface of their planet and detonated them. However even humans aren't that stupid and tests soon moved underground.
The first test took place just as the Driv were approaching to claim earth for themselves. The human's bomb was buried in a deep hole, with a 10 ft plug of concrete on top. Covering the hole was a large manhole cover. The bomb was detonated, turning the concrete into rapidly expanding plasma. This concrete-plasma slammed into the manhole cover with unimaginable force and fired the manhole cover into space at 6 times the escape velocity of earth. The manhole cover easily punched through the first few Driv ships it encountered before disappearing into deep space.
The Driv then decided to gather some more info on earth and found that similar anti space weapons dotted the streets of almost every major population center on earth. They then hi-tailed it back to their planet.