This is a brief review on the work done recently. It is shown that the global constraints of Gauss' law ensure that the vacuum angle must be quantized in gauge theories with magnetic monopoles. Our quantization rule is given as $\theta=0$, or $\theta=2\pi N/n~ (n\neq 0)$ with integer n being the relevant topological charge of the magnetic monopole and N is an unfixed integer in this approach. This theoretically confirms further the conclusion originally proposed by the author$^1$ that the strong CP problem can be solved due to the existence of magnetic monopoles, the fact that the strong CP-violation can be only so small or vanishing may imply the existence of magnetic monopoles, and the universe is open.
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