CREATE TABLE table_37431 ("Round" FLOAT, "Player" TEXT, "Position" TEXT, "Nationality" TEXT, "College/Junior/Club Team" TEXT)

What is the round of the defense player from the United States?
SELECT "Round" FROM table_37431 WHERE "Position" = 'defense' AND "Nationality" = 'united states'
SELECT "Round" FROM "table_37431" WHERE "Nationality" = 'united states' AND "Position" = 'defense'