From the Mystic Stamp Company Website:
Early American poetry of the 17th century was largely British influenced. By the mid-20th century, American poetry had become such a distinct style, British poets looked to the U.S. for inspiration.
American poetry moved away from the traditional style in the mid-1800s, led by Walt Whitman and Emily Dickinson. Their use of free verse, emotion, obscurity, and irony shaped 20th century poetry.
At the turn of the 20th century, poets rejected traditional forms and focused on fragmentation, ellipsis, allusion, juxtaposition, irony, shifting perspective, and mythological parallels. Significant poets of the time included Wallace Stevens, William Carlos Williams, and E.E. Cummings.
Following World War II, a new generation of poets emerged, largely influenced by Wallace Stevens. Among them were Elizabeth Bishop and Theodore Roethke. The end of the war also brought a flood of new styles, including Confessional and Beat poetry, which strongly influenced Sylvia Plath. This era also saw the emergence of the Black Mountain poets, among them Denise Levertov. The 20th century saw numerous other styles, including focus on the works of African-Americans like Gwendolyn Brooks and Robert Hayden. America also welcomed and embraced writers from other nations, including Joseph Brodsky, who became the first foreign-born U.S. Poet Laureate.
From humble beginnings, American poets developed a distinct new style that has continued to evolve into a new century.