An M-theory flop as a large N duality

We show how a recently proposed large N duality in the context of type IIA strings with $\mathcal{N}\text{=1}$ supersymmetry in 4 dimensions can be derived from purely geometric considerations by embedding type IIA strings in M-theory. The phase structure of M-theory on $G_2$ holonomy manifolds and an $S^3$ flop are the key ingredients in this derivation.