JohnDubYa said:
In no way does this imply that the security of the United States is the only reason to challenge Communist invasion. In fact, the doctrine repeatedly points out an emotional appeal to help those that being attacked by totalitarian regimes: "... unless we are willing to help free peoples to maintain their free institutions and their national integrity against aggressive movements that seek to impose upon them totalitarian regimes."

I see you couldn't stand to leave in the part of the quote that talks approvingly of the UN and the US' role in founding it...
Also, if you feel it necessary to base an argument on just one part of a quoted sentence, where the complete sentence pretty much makes the opposite point, it's generally best not to also include the complete sentence in your post.

The full sentence reads:
"We shall not realize our objectives, however, unless we are willing to help free peoples to maintain their free institutions and their national integrity against aggressive movements that seek to impose upon them totalitarian regimes." [Emphasis added]
The above is even more true when the document being quoted has sections that would make your point better, e.g.:
"When forces of liberation entered Greece they found that the retreating Germans had destroyed virtually all the railways, roads, port facilities, communications, and merchant marine. More than a thousand villages had been burned. Eighty-five percent of the children were tubercular. Livestock, poultry, and draft animals had almost disappeared. Inflation had wiped out practically all savings. As a result of these tragic conditions, a militant minority, exploiting human want and misery, was able to create political chaos which, until now, has made economic recovery impossible."
This is all beside the point however. The emotional appeals are more or less window dressing. Truman knows perfectly well a sentimental argument isn't going to win the support of congress. The real argument starts with:
"I am fully aware of the broad implications involved if the United States extends assistance to Greece and Turkey, and I shall discuss these implications with you at this time."
After this point, emotional statements are consistently punctuated with references to national security and what is now called the domino theory. This is true of the original quote I gave, and equally true of Truman's concluding statement:
"The seeds of totalitarian regimes are nurtured by misery and want. They spread and grow in the evil soil of poverty and strife. They reach their full growth when the hope of a people for a better life has died. We must keep that hope alive.
The free peoples of the world look to us for support in maintaining their freedoms. If we falter in our leadership, we may endanger the peace of the world -- and we shall surely endanger the welfare of our own nation."
None of this is a statement about Truman's motives—I just note that he skillfully renders his argument in a fashion suited to the context and subject. I don't have a good enough sense of US politics at the time to judge whether his success with this particular policy was due more to that skill, or to the overall idea being "in the air" at the time.
Also, nothing I have said is intended to imply that sympathy for an invaded or oppressed people is not a worthy reason for action, just that it's not the type of reasoning that would have driven US foreign policy in the mid 20th century.
So far your statements here have mostly conveyed a strange taste for sentimental policy arguments, and a fairly foggy perception of the Cold War mindset. Neither of these impressions seem especially likely to be accurate—you are welcome to correct them.