Answer: The right answers are World War I, World War II and the Cold War.
Explanation: Just to elaborate a little on the answer, it can be added that the Vietnam War (1955-1975) must be excluded from the list, since it was fought between South Vietnam and its ally, the United States, and North Vietnam, although a few other countries were also involved. World War I (1914-1918) and World War II (1939-1945) did affect the whole world, even though the former initially only involved Serbia and Austria-Hungary and the latter originally only confronted Germany with France and the United Kingdom. As per the Cold War (1947-1991), it was a conflict primarily between the United States and the Soviet Union, and their respective allies, but it eventually permeated the while world. Many colonial regimes fell as a result of this worldwide conflict. In her poem, poet Muriel Rukeyser (1913-1980) is stressing that she lived in the first century that witnessed the emergence of those worldwide and devastating conflicts and that, in spite of the horror, she hopes for a better future.