The Modernism literary movement was heavily influenced by 20th century atrocities, such as world wars I and II. These wars left the world "shell-shocked" and cynical and asking the question, what is to become of the world and humanity? While the Romantic era saw the world as being full of "progress and growth, the Modernist intelligentsia [saw] decay and a growing alienation of the individual"; they saw decline of civilization (The Literature Network, "Modernism"). They saw everything about the modern world as being cold, impersonal, and antagonistic, such as machinery and capitalism. Hence, major characteristics of Modern literature were self-reflection and cynicism.
In William Butler Yeats's brief love poem "A Drinking Song," we see modernism in the speaker's cynical questioning of both love and truth. In saying, "... love comes in at the eye," he is saying that love is based only on physical appearances; love is merely an illusion just as looks are an illusion. In saying, "That's all we shall know for truth," he is questioning the very existence of truth. If love is only an illusion based on physical appearances, then even love cannot be called true. If there is no truth in love, then truth can't really be known at all and possibly doesn't even exist at all. Hence, the speaker ends by saying, "I look at you, and I sigh," because he recognizes the beauty of the person before him and that both beauty and love is a futile illusion.