Refer to the passage. "4. As a result, it is indicated that the future may see a time when such a weapon may be constructed in secret and used suddenly and effectively with devastating power by a willful nation or group against an unsuspecting nation or group of much greater size and material power. With its aid even a very powerful unsuspecting nation might be conquered within a very few days by a very much smaller one; although probably the only nation which could enter into production within the next few years is Russia.

5. The world in its present state of moral advancement compared with its technical development would be eventually at the mercy of such a weapon. In other words, modern civilization might be completely destroyed.

6. To approach any world peace organization of any pattern now likely to be considered, without an appreciation by the leaders of our country of the power of this new weapon, would seem to be unrealistic. No system of control heretofore considered would be adequate to control this menace.”

Excerpt from a memorandum discussed with the US president regarding the development and use of the atomic bomb, 1945

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