Dear Pat,
From the point of view of Generational Dynamics, that's actually not
the right question. The fourth turning is not triggered by events.
The fourth turning is triggered by a mostly unvarying and inexorable
generational change, and it's the generational change that triggers
events. (The "regeneracy" is triggered by events, but the fourth
turning is not.)
The fourth turning occurs when the previous Artist generation
completely loses influence in society, and that happens almost
unvaryingly and inexorably about 58 years after the climax of the
preceding crisis war. The Revolutionary War climaxed in 1782, and 58
years later is 1840.
In our own time, we chose not to go after Saddam Hussein in 1991, and
to have no major reaction to the World Trade Center bombing in 1992.
But we declared war against Afghanistan in 2001, and launched a ground
invasion of Iraq in 2003. The thing that changed was the end of the
preceding Artist generation, and the rise of the new Nomad generation.
In the 1840 time frame, here's an introductory description to
the Mexican-American war:
<QUOTE>Causes of the Mexican-American War
Texas gained its independence from Mexico in 1836. Initially, the
United States declined to incorporate it into the union, largely
because northern political interests were against the addition of
a new slave state. The Mexican government was also encouraging
border raids and warning that any attempt at annexation would lead
to war.
Nonetheless, annexation procedures were quickly initiated after
the 1844 election of Polk, who campaigned that Texas should be
“re-annexed” and that the Oregon Territory should be
“re-occupied.” Polk also had his eyes on California, New Mexico
and the rest of what is today the U.S. Southwest. When his offer
to purchase those lands was rejected, he instigated a fight by
moving troops into a disputed zone between the Rio Grande and
Nueces River that both countries had previously recognized as part
of the Mexican state of Coahuila."<END QUOTE>
http://www.history.com/topics/mexican-american-war
So what changed between 1836 and 1844? The same thing that changed
between 1991 and 2003 -- the end of the preceding Artist generation,
and the rise of the new Nomad generation, and that's the fourth
turning, or Crisis Era.