To what extent was US's use of atomic weapons on Japan in the Second World War justified?

Considering the profound impact of the United States use of the atomic weapon on modern history, the historical debate on whether or not the US is justified in employing it near the end of the second world war has been unceasing. Some historians argue that the United States was justified based on the relentless resistance United States faced in their earlier invasion and takeover of Okinawa. Evidence of the fierce Japanese resistance includes the infamous adoption of Kamikaze attacks on United State's fleet and " death before surrender" mentality Japanese army and civilians displayed. Furthermore, the fact that Japanese deliberately ignored Potsdam ultimatum issued days before US deployment of the atomic bomb which urged the unconditional surrender of Japan days before US's deployment of atomic bomb aids in the case that Japanese government are determined to resist a Japanese mainland invasion to the last men. In this sense, the US's use of atomic weapons could be seen as justifiable prevention of massive casualties on both sides based on the extent of resistance the US faced in Okinawa and Japan's clear refusal to surrender. 

On the other hand, some historians argue that US deployment of atomic weapons was motivated by political reasons and thus not justified in its cause. This particular view highlights how the US's decision to deploy atomic weapons eerily coincides with the Soviet Union's launch of invasion of Japan, which presumably would allow for a faster takeover of Japan with fewer casualties. The argument follows that the US is fearful of Japan falling into the hands of the Soviet Union after the invasion and become a communist state, and hope to force Japan to surrender before so by employing the weapon of unprecedented scale that would incur massive civilian casualties. In turn, the deployment of atomic weapons is motivated by the maintenance of US interests in Asia, whereby tens of thousands of innocent civilians are sacrificed for this cause. However, this line of thought is limited by the fact that Soviet invasion of Japan is a designed part of the war effort by the allies and is actually encouraged by President Roosevelt in numerous occasions during the war. Moreover, it is doubtful that the extent of suspicion and tension between the US and the Soviet Union at this moment in time would be sufficient for the US to use atomic weapons in fear of Japan turning communist as a result of a Soviet invasion. In conclusion, the US's use of atomic weapons in Japan is to a large extent justified by virtue of minimizing casualties on both sides in case of a land invasion and the various limitations to the argument that it was politically motivated.