Ron Paul's "foreign policy"--if one can call neo-isolationism/non-interventionism a foreign policy--rests on naive assumptions that humans, by nature, are good:
1. All countries governments are benign and they aspire to peace.
2. All countries' share the same interests making conflict unnecessary.
3. Power is irrelevant.
4. Allies bring no value-added, only risks.
Based on those assumptions, he believes a neo-isolationist/non-interventionist foreign policy in which the U.S. would sever its alliances would lead to permanent peace and friendship with all.
The reality across history is starkly different. Not every nation desires peace. Interests, even critical ones, can clash and sometimes the clashes are so deep and irreconcilable that diplomacy is not viable. Power matters when it comes to national security. Weakness has often been exploited. Allies can work together cooperatively to advance the common interests they share.
In the run-up to U.S. involvement in WW II, the U.S. "tested" a lot of what Paul espouses today. The U.S. still found itself attacked and embroiled in WW II. The Cold War provides a vivid illustration that alliances (NATO in this case) can bring enormous value and promote stability.
In the end, Ron Paul sees the world in 1D. In reality, the world is far more complex and dynamic. His simplistic foreign policy would abdicate U.S. interests, abandon U.S. allies, and put the nation on a long-term path that undermines its security.