The third thing on my checklist is relations with neighboring friendly societies that may prop up a society. And if that friendly support is pulled away, that may make a society more likely to collapse. In the case of the Greenland Norse, they had trade with the mother country -- Norway -- and that trade dwindled: partly because Norway got weaker, partly because of sea ice between Greenland and Norway.
在我的清单上的第三点是： 与周边友邦的关系，这些友邦可以提供必要的援助。 而一旦这种来自友邦的援助终止，通常会使 这个社会崩溃。在这个案例中， 他们一直在与母邦，挪威，进行贸易往来， 然而，一方面因为挪威逐渐衰弱， 一方面因为两地航道间的海水渐渐结冰，这种贸易与日俱减。