The poem of the mind in the act of findingWhat will suffice. (1-2)
Well there wouldn't be any reason to find "What will suffice" if everything in the world were already good. So that means that right from the get go, Stevens is telling us that normal human life isn't satisfying enough on its own. It needs poetry and literature to help make up for the fact that life is basically unsatisfying in some way.
It has not always hadTo find. (2-3)
The kind of dissatisfaction that drives modern poetry hasn't always been around. At some point in the past, people might have actually felt pretty darn good about their lives and the universe. Religion held total sway, everyone knew their place, and there wasn't much that anyone could question about how the world worked. But after some people started questioning things like religion, monarchy, and the basic meaning of life, things changed. People started feeling a little insecure, and began to wonder if the universe was just completely indifferent to human concerns. That's not a comforting thought, and definitely something that'll cause a bit of dissatisfaction.
It has to think about war And it as to find what will suffice. (9-10)
The gruesomeness of the two World Wars no doubt shook people's sense of satisfaction with the world. After all, who could be happy with a world that used all of our great scientific advancements to fill the fields of Europe with tens of millions of bodies? Modern poetry, though, has to confront the horror of war if it wants to find a way to satisfy people about the state of the world.