The reason people are willing to buy US government debt is because they are confident the US government will be able to pay them back, with interest. Why are people confident in the US government's ability to pay...and where does this money come from? The strength of the growing US economy and the money the government will derive from taxes on this growth.
Or conversely, let's look at what the US government spent that money on: supplying the war machine via US industrial capacity. That industrial capacity wasn't created out of thin air. Take a look at Detroit for example. Absent the existing industrial capacity enabled by the automotive industry, the allies would likely have not won the war.
Both the ability of the US government to deficit spend, and the things they spent that money on, were a direct result of the strength of the US's private enterprise.
If you include the millions who died in various famines as well as those that were executed directly by Stalin, the number of victims of the USSR has been estimated to be anywhere between 6 and 15 million people.
The Soviet Union has to be the worst counterpoint to anybody arguing against private enterprise.