It still seems to me that the most pressing problem facing the US (in terms of national security) is the potential for terrorist attacks. I do not foresee any actual nation attacking the homeland of the United States in the near future and so that leaves terrorists as the major threat to Americans within the US. Now, does that mean our president needs to "respond?" I'm not sure. Our last two presidents have been responding, but I suppose that further responses are needed since I do not believe that we are safe from attacks.
I'll cite two other potential risks. Long term, there may be a risk of China becoming too strong. But they do not have the military power to threaten major US interests at the moment. We may need to respond to their economic and other "soft" power, though.
Finally, if the drug wars in Mexico start to spill over into the US, that could be a serious threat to certain areas of the country.