Before including the Russian BMP-1 troop carrier, one might want to consider how much the first use of the airplanes have changed the battlefield. No battle since WWI has been fought without airpower. If I had to narrow it to one aircraft I would consider the Me 262. Most fighters since WWII are based on its jet powered swept wing design. Also one should consider the machine guns of WWI which replaced the bolt action rifles. The submarine, especially the nuclear ballistic sub which can stay on station almost indefinately and strike from hundreds of miles away without being detected, changed the way the USSR thought about a first strike survivability against the US. And finally the Little boy atomic bomb. The world has been and will always be under the cloud and fear of nuclear weapons because of this one device. Judged against these weapons, the BMP-1 making the list at number 3 or even at all seems a little ridiculous.