Final answer:
Electrostatic repulsion is the natural barrier that prevents two protons from combining, which is overcome by the strong nuclear force within the nucleus.
Step-by-step explanation:
The natural barrier that tries to prevent two protons from combining is known as electrostatic repulsion. Protons, being positively charged, repel each other due to the Coulomb force. However, within the nucleus, this repulsion is overcome by the strong nuclear force, which acts at very short distances and is powerful enough to keep the nucleus together including protons and neutrons. Quantum mechanical tunneling is another phenomenon that allows protons to pass through this repulsive potential barrier under certain conditions. But fundamentally, the electrostatic repulsion is what naturally prevents protons from combining easily.