Based on this letter, Albert Einstein wanted the United States to -

Fearing for his safety, he and his wife, Elsa, immigrated to the United States on October 17, 1933, where Einstein accepted a position at the Institute for Advanced Study in New Jersey. ... After World War II, Einstein advocated nuclear disarmament and international cooperation. He died in 1955

Explanation:

crop rotation kepy different plants in the soil instead of once constintly. when switching out plants you have different nutrients being absorbed at different speeds and different roots growing differently therfore the spoil and water flow have to adapt and carry fresh nutrients to carry the constant changes that were going on in the soil.