The great depression and prohibition reinforced this isolation. The North American contenent was not the center of the arts for the first half of the century, although a tradition of fine art experimentation started in New York near the turn of the century. After World War II, arts in North America flowered. Global communication expanded in an awe inspiring way. The result was that by the end of the 20th Century there is a global arts community.
Nobody does anything of significance without global implications.
By this I don't mean art of significance doesn't get created in the dark corners with little if any publicity. But even the most private artist is affected by the global communication in place. So one is influenced by what ever is publicised anywhere in the world. It no longer makes little sense to view art since WW II on a regional basis. So art for the second half of the 20th Century is just organized alphabetically by artist name, independent of country of origin.