Visiting the National World War II Memorial in Washington is a sobering experience. The cascade of gold stars adorning the walls are a heart-rending depiction of the 400,000 American service members who died in both the Pacific and European theaters of war.

Each of the 4,048 stars represents 100 American deaths – sons, fathers and brothers who never came home. Imagine the human tragedy if the number of gold stars were doubled, which would have been the result of an Allied invasion of Japan.

U.S. government wartime casualty assessments provide a chilling reminder of the human cost of an invasion had President Harry Truman decided not to drop the atomic bombs on Hiroshima and Nagasaki. The lowest number of estimated fatalities appears to be 267,000. Other assessments go as high as 500,000 or 1 million fatalities, with many more that number being wounded.

As horrendous as these predictions were, they may well be underestimates. Post-war access to captured Japanese documents and senior Japanese military leaders indicate Japan had greater military forces available to defend the homeland than U.S. officials predicted.

Truman said his goal was to "shorten the agony of war, in order to save the lives of thousands and thousands of young Americans." Imagine U.S. public outcry had Truman instead opted for an invasion, prolonging the war and increasing casualties, when it became known another option had been available.

In his memoir, Truman wrote that after Japan rejected another plea for surrender, he had no qualms about his decision to drop the bombs "if millions of lives could be saved … I meant both American and Japanese lives."

As casualties from an invasion escalated, who could say that atomic weapons would not have been used on other targets in Japan? It has been suggested that the devastation caused by the 1945 bombings led both the United States and Soviet Union to refrain from using nuclear weapons during numerous Cold War crises.

Emperor Hirohito announced to his subjects that he based his decision to end the war on the "new and most cruel bomb … Should we continue to fight, it would ... result in an ultimate collapse and obliteration of the Japanese nation."

Given the tenacity of Japanese defense and high civilian fatalities during the battles for Iwo Jima and Okinawa, it is likely that allied invasion of Japan would have caused millions of Japanese casualties.

In addition, there are estimates that 100,000 to 250,000 non-combatants in occupied Asia would have died for every month that the war was extended. There are also reports that Japanese commanders had orders to execute all 400,000 allied POWs if an invasion of the Japanese homeland occurred.

Had the Pacific War been extended, the Soviet Union would likely have invaded and occupied the northern half of Japan. Any student of life in the Soviet Union and occupied Eastern Europe knows how brutal life would have been for anyone living in Soviet-occupied Japan.

Hiroshima and Nagasaki were tragedies, as were all other deaths from World War II. But the decision to drop the bombs averted the even larger tragedies that would have resulted from a full-scale invasion of Japan.