How can you tell if an argument is valid?

cabeza de vaca was an spanish explorer and n his long exploration of 11,000.km. from florida to present-day mexico, he went through a series of calamities and setbacks that only added to his deeds. he applied his religious creed with the humanist idea, always current and universal, that all men were equal before god and destiny without any discrimination. as an explorer, he became a slave to the tipinoga, then his shaman pointer, later and after an operation of heart to an indian (from which both came alive) he achieved stardom and became a hero followed by a crowd that professed unusual respect and admiration.

1. read the following excerpt from franklin d. roosevelt's "fireside chat 19":

on the other side of the picture, we must learn also to know that guerilla warfare against the germans in, let us say serbia or norway, us; that a successful russian offensive against the germans us; and that british successes on land or sea in any part of the world strengthen our hands.

remember always that germany and italy, regardless of any formal declaration of war, consider themselves at war with the united states at this moment just as much as they consider themselves at war with britain or russia. and germany puts all the other republics of the americas into the same category of enemies. the people of our sister republics of this hemisphere can be honored by that fact.  

the true goal we seek is far above and beyond the ugly field of battle. when we resort to force, as now we must, we are determined that this force shall be directed toward ultimate good as well as against immediate evil. we americans are not destroyers — we are builders.

analyze the effectiveness of this excerpt. how does it function within the speech as a whole? how does the structure of the excerpt to convey the speech's purpose and key points? support your argument with evidence from the speech. (10 points)