What is the purpose of the phrase "secure the Blessings of Liberty to ourselves and our Posterity in the U. S. Constitution's preamble?
We the People of the United States, in order to form a
more perfect Union, establish Justice, insure domestic
Tranquility, provide for the common defence, promote the
general Welfare, and secure the Blessings of Liberty to
ourselves and our Posterity, do ordain and establish this
Constitution for the United States of America,
-U. S. Constitution, Preamble!
'U. S. Constitution, Preamble, June 21, 1788, National Archives and Records Administration,
A. To guarantee that the U. S. government will have separation of
powers
B. To place strict limits on the power of the U. S. federal government
O C. To show that the people hold all political power in the country
OD. To state that the government is expected to protect American
freedoms

The phrase 'to secure the blessings of liberty to ourselves and posterity' appears in the Preamble to the Constitution. The last word is posterity, meaning children and future generations, rather than prosperity or wealth.

Explanation:

the right answer is the end of world war ii

after the end of a global wwii all the nations involved, in an attempt to create an organization to maintain world peace. this agreement would work settling international disputes via negotiation and arbitration instead of leading the issues escalate to another war.

since most of current immigration into the u.s., both legal and illegal, is coming from latin america. it would be very difficult to imagine or rationalize a country my fellow citizens are trying desperately to flee into, would pose a threat to my prosperity or freedom. since the u.s. has a long history of intervention, but no record of permanent occupation, i would not feel oppression is the main objective.  

the largest free trade nation, along with the largest gdp in the world wanting to expand it's economic policy on us, and including us, could only be positive. the u.s. government has no interest in, or way of simply forcing american culture on us. and has no track record of it. other nations that adopted american social culture, or "americanization" did so voluntarily, mainly through the choice of allowing american businesses and u.s. citizens to come into their nations, as free operatives.