A seemingly throwaway line in All-Star Squadron #35, at the conclusion of a confrontation between the Freedom Fighters from Earth-2 battling their world's Baron Blitzkrieg on hero-less Earth-X, whilst the Spectre kept both Earths from colliding together due to a cosmic in-balance caused... by the incursion of these mystery men onto a world not their own!
The three who remained behind on Earth-X were Uncle Sam, Black Condor and the Ray; while Nazi Troopers 1, 2 and 3 returned with the Baron to Earth-2. However, previously Sam brought other heroes to Earth-X, all of whom (save for Hourman) remained until their premature deaths:
According to the law of conservation of mass, also known as the principle of mass/matter conservation, is that the mass of a closed system will remain constant over time. The mass of an isolated system cannot be changed as a result of processes acting inside the system. A similar statement is that mass cannot be created/destroyed, although it may be rearranged in space, and changed into different types of particles.
So the mass of these metahumans was removed from one closed system, or universe, and transferred into another. How did the multiverse compensate for this unauthorized transfer? Well another incursion happened shortly thereafter when several Nazi troops followed Midnight and Dollman back from the Nazi's Earth-X to the heroes' Earth-Two:
In the third panel, if you squint you can make out ten soldiers on the opposite rooftop from Midnight, one leaning over it and two falling presumably to a rough landing. That's thirteen, so five of these would cancel out the five late heroes Red Torpedo, Magno, Miss America, Neon the Unknown and the Invisible Hood. Another would cancel out Midnight, who would return to Earth-X shortly thereafter accompanied by seven aircraft aces, the Blackhawks. So the total number of soliders and Fighters compensate for the transfer of mass. However, there were others whom Uncle Sam brought with him to this new Earth during the Crisis on Infinite Earths. How would Earth-Two compensate for their loss? We'll dissect this and other cross-world quandries in the upcoming series.