It really isn't. The US debt is about 1X GDP. Japan's debt is about 2X GDP...both great nations.
According to the World Bank, every percentage point above 77% for the GDP-to-Debt ratio costs a developed country 1.7% of economic growth, so we are already crippling our economy with our debt. Now let's look at Japan. Japan has the highest ratio of public debt to GDP of any developed nation in the world, and their economy is famous for its 20 years of stagnation despite a hard working, highly advanced, highly trained, and highly educated work force. So comparing us to them and saying it's all okay is like saying a coke addict is fine because it's not as bad as a meth addict.
Joint Chiefs of Staff Admiral Michael Mullen and Defense Secretary James Mattis have both separately stated that the single largest threat to US security is... Russia? ISIS? Iran, China, North Korea? Nope... they said single largest threat to US security is the national debt.
Director of National Intelligence Dan Coats has said something similar. The largest internal threat to US security is the national debt.