Final answer:
Walt Whitman was the first American poet to receive worldwide critical acclaim, particularly with his influential work 'Leaves of Grass' published in 1855.
Step-by-step explanation:
The first American poet to win worldwide critical acclaim was Walt Whitman, whose groundbreaking use of free verse and celebration of the common language of the common man revolutionized poetry. Whitman first garnered public attention with the publication of Leaves of Grass in 1855. Robert Frost, another iconic American poet, achieved significant critical acclaim during his lifetime and won the Pulitzer Prize four times between 1924 and 1943. Frost's poetry resonated with the nation much like Norman Rockwell's artwork, as he became an American icon. Although both poets are celebrated, Whitman is often credited as being one of the first American poets to achieve global recognition for his bold and innovative style.