G-Module is Irreducible iff no Non-Trivial Proper Submodules

Theorem
Let $\left({G, \circ}\right)$ be a finite group.

Let $\left({V, \phi}\right)$ be a $G$-module.

Then $V$ is an irreducible $G$-module $V$ has no non-trivial proper $G$-submodules.

Necessary Condition
Assume that $V$ is an irreducible $G$-module, but it has a non-trivial proper $G$-submodule.

By the definition irreducible, its associated representation is irreducible.

Let this representation be denoted $\tilde \phi = \rho: G \to \operatorname{GL} \left({V}\right)$).

In Correspondence between Linear Group Actions and Linear Representations it is defined as:
 * $\rho \left({g}\right) \left({v}\right) = \phi \left({g, v}\right)$

where $g \in G$ and $v \in V$.

Since $V$ has a proper $G$-submodule, there exists $W$ a non-trivial proper vector subspace which $\phi \left({G, W}\right) \subseteq W$ and so $\rho \left({G}\right) W \subseteq W$.

Hence $W$ is invariant by every linear operators in $\left\{ {\rho \left({g}\right): g \in G}\right\}$.

By definition, $\rho$ cannot be irreducible.

Thus we have reached a contradiction, and $V$ has then no non-trivial proper $G$-submodules.

Sufficient Condition
Assume now that $V$ has no proper $G$-submodules, but it is a reducible $G$-module.

By the definition of reducible $G$-module, it follow that its associated representation is reducible.

Let this representation be denoted $\tilde \phi = \rho: G \to \operatorname{GL} \left({V}\right)$).

From the definition of reducible representation, it follows that there exists a vector space $W$ of $V$.

This is invariant under all the linear operators in $\left\{ {\rho \left({g}\right): g \in G}\right\}$.

Then:
 * $\phi \left({G, W}\right) = \rho \left({G}\right) W \subseteq W$

which is the definition of a $G$-submodule of $V$.

By our assumption, $V$ has no non-trivial proper $G$-submodules.

Thus we have reached a contradiction and $V$ must be then an irreducible $G$-module.