

Yes and per article 5 the US would have to act as if they attacked themselves. Since this is quite silly, an attack of the US against Greenland would most probably either lead to the US having to leave NATO (I don’t know, if there is a way to expel a country from NATO) or NATO dissolving completely.
As other people already named it, personal interaction is one big factor. Being in a friend group, learning together and trying to achieve things together greatly enhances the chances to complete the studies.
Also this is only possible for lectures and most seminaries. Outside of social science and humanities you usually have some kinds of hands on or lab courses, which of course can’t be done online.