The speaker begins by harshly criticizing America. The country has, the speaker implies, beat him down him with its oppressive violence. Rather than nourishing his body and spirit, it has force-fed him "bread of bitterness." This metaphor suggests that bitterness—or resentment and great difficulty—is something the country imposes on him regularly, as if it's a daily diet of bread.
Although America's Declaration of Independence honors the right to "life, liberty, and the pursuit of happiness," the country attacks the speaker like a "tiger's tooth" piercing his neck. This second metaphor indicates that the speaker feels genuinely attacked by the country, as if the United States has purposefully hindered his vitality by taking away his "breath of life." The hard alliteration and consonance in phrases like "bread of bitterness" and "tiger's tooth" help express the speaker's anger about this; they also hint at the violent, merciless nature of the nation.
These opening lines also personify the U.S. as a woman ("she"), following an old-fashioned convention of referring to countries by female pronouns. In patriotic literature, the feminized country is typically portrayed as an ideal woman and the object of the speaker's love. Here, the speaker flips that convention by portraying America as a tormenting woman—a cruel mistress—whom he loves in spite of her violent hatred.
The first three lines also establish the sonnet's use of iambic pentameter, a meter in which each line contains five iambs, or feet consisting of an unstressed syllable followed by a stressed syllable (da-DUM). This is the most common metrical pattern found in the sonnet form—a form most often used to express complicated kinds of love. From the first lines, then, McKay uses this poetic convention while putting a new spin on it, since "America" is—in many ways—a love poem that expresses a very deep, complex, and even disastrous relationship between a man and his country.