This is from Leslie Gelb on NPR on THE POST film and THE PENTAGON PAPERS (the writing of which he oversaw). Specifically, he believes the real story is not that The Powers That Were in the USA lied but what they believed true.
But the main reason we got involved in the Vietnam War was because of what we believed, namely that we were in a mortal struggle with communism - Soviet Union and China; that the domino theory applied, that we if we allow North Vietnam to conquer South Vietnam, the dominoes would fall throughout Asia and the rest of the world.Add to this theory subsidiary theories: JFK on the Green-Beret approach to counter-insurgency, Robert McNamara on rational ways to run a war. And I'll add (again) that people could argue that the USA won the Cold War in Vietnam precisely by showing how much money we were willing to spend, how many S.E. Asians we were willing to kill, how many of our own troops we were willing to have killed, captured, wounded, and/or maimed to stop even so minor a "domino" as Vietnam to fall.
That is a non-figurative meaning of "bear any burden" and a possibility for "ask not what your country can do for you, ask what you can do for your country" — including putting your body on the line to aid in a Great Game of power politics, and to keep your country from losing its first war.