1) The author argues that Americans should care about Brexit because it could negatively impact peace and prosperity in Europe by fueling nationalist movements and potentially causing countries to leave the EU. 2) A breakdown of the EU single market through Brexit would make trade and travel across Europe more complicated and costly for both European countries and American companies doing business in Europe. 3) Brexit could weaken the UK's role as a gateway for US businesses to access the EU single market, negatively impacting the $558 billion US companies have invested in the UK.