The quote "From Stettin in the Baltic to Trieste in the Adriatic an iron curtain has descended across the Continent" is referring to the Division of Europe during the Cold War. This phrase, famously used by Winston Churchill in a speech in 1946, describes the political and ideological divide between Western countries, underpinned by democratic and capitalist principles, and Eastern countries, which were influenced by the Soviet Union and communism.
Regarding the cartoon explaining a reason for joining NATO, the correct interpretation would be that Countries wanted to be a part of a military alliance. NATO (North Atlantic Treaty Organization) was established as a collective defense organization during the Cold War, primarily to counter the threat posed by the Soviet Union.
According to the quote, the US planned to approach world affairs after WWII by advocating for a policy of supporting free peoples resisting oppression, and their assistance would primarily come through economic and financial aid. This reflects the essence of the Truman Doctrine, which aimed to contain communism and support nations resisting outside pressures or armed minorities.