What had to be gone first, what was in place already and what was new that brought this division about?

Over fifteen million Americans served in the military in WWII.  College graduates usually became officers.  It was a source of civil behavior and meritocracy.  The rest of society accepted civility as a military necessity.

When the war was over the civility that had been needed and created after the end of the American Civil War with civic groups, dissolved. Membership in the Masons, Odd Fellows, Kiwanis, Lions, scouts and others shrank to insignificance.

More importantly, the meritocratic military officers entered the labor force and rose to the top ranks of the fifty major corporations; they formed hundreds more corporations and displaced the source of power for the American hereditary elite. By 1960 the hereditary elite was so disrupted and displaced that it no longer published the Black Book of membership (actually red) and the debutante balls sponsored by local hereditary elites were replaced with non-profit sponsors.  

America entered a period of rapid corporate expansion, globalization and technological innovation.

The absence of the hereditary elite left the second most powerful institution in control in urban areas: labor unions. Labor unions came quickly to control the politics of every metropolitan area to this day.  Unions are seniority-oriented, very rule based and are deeply anti-meritocratic.

Two other lesser institutions gained power and prominence after the fall of the hereditary elite and civic organizations.  Academia which had been Marxist for most of the 20th century took over the intellectual world and the media. 

Prison values took over fashion in pants to the knees and rap which uniquely had performers being murdered wantonly. Big bands and rock groups had never been homicidal.

All of this allowed a long simmering division in America to become visible and important as populations moved; white families left bused school cities for the suburbs and young Lefties moved to urban centers after college.

Then came the most powerful force to hit the vulnerable United States and Israel, the new social media on the Internet.  The self-fulfilling mirror.

The extraordinary impact of the Internet is not obvious.  The Internet and social media allowed individuals to expand their personal and political worldview to prominence and full blown significance.  Millions of people who played little or no role in prior civic life or politics all of a sudden had a vehicle to publish and voice their opinions in a semi public way and find allies to align with.

Voila, we had a divided America and a similar Israel where hatred could be expressed openly and civil behavior was drastically diminished.

A whole new phenomenon where individuals expanded their opinion persona and found sympathizers among strangers.

Welcome to divided America. Welcome to angry Americans.