The way I've always heard the story told, IBM actually kept the patent peace in the industry for decades. They have thousands of patents going back to the foundational era of computing, and warehouses full of prior art. They also operated for a decade or so under harsh anti-trust scrutiny, which limited the viability of "offensive" patent use. Instead, if some upstart threatened to corner off key new technology, IBM was always ready to fire back with some "method for utilizing a spinning disk to store structured records" patent. Cross-licensing agreements among the big players were the norm, not patent fights (even the big Microsoft/Apple dustup over the GUI was over copyright, not patents).
I see evidence for this folklore in that the biggest technology patent wars we've seen in recent years have focused around wireless, which one might consider the first major new computing technology that IBM didn't take a big part in developing.