Probably the same way they fought undeclared wars against France (the quasi war) and later the Barbary Pirates. Built a defence, in their case create a Navy, reestablish the USMC, and take the fight to the enemy. In Frances case off their coast and the Carribean, the pirates in the Mediterranean. Both wars fought by the founders, undeclared, and fought over trade far from America. Taking the fight to the enemy is an old American tradition, not always followed. And sometimes we make mistakes, which are unrelated to the Constitution or the founders as paulestinians would have us believe.
With a national debt of $16 trillion, almost incalculable unfunded government and private debt, a declining industrial base, and the worst educational system of any Western democracy, the United States cannot sustain the international role it has for 70 years. Irrespective of Ron Paul's unrealistic vision, this nation will need several decades to rebuild the sort of power needed to become preeminent once again. We have tried to provide guns and butter. Given the choice, the electorate will likely choose the latter. Realistically, the government will be able to provide neither sooner than later.