There are species on different continents with very similar genetic makeups that could have shared common ancestors. This would only be possible if continental drift had happens as there is no way some of these species could have derived from common ancestors.
I'm assuming you meant subsistence farming? If so then yes, it would be a large problem because they would then not have enough food to feed themselves.
I would say that the US and the Soviet Union at the end of WWII withdrew their troops as soon as a demilitarized zone was created. Also, I am sure they would require a peace treaty to be signed with Germany before withdrawing troops.
