Why did US military planners consider the development of an atomic weapon a necessary alternative to previous conventional warfare strategies?

They believed that the United States needed to develop an atomic weapon to secure a military alliance with Russia.
They believed that the United States needed to develop an atomic weapon to win the war against Germany, despite moral concerns.
They believed that the only way the United States could swiftly end the war with Japan was by developing an atomic weapon.
They believed that it was urgent for the United States to keep up with advancements in Japan’s military technology by developing an atomic weapon.