From the wikipedia article on Cartan decompositions, a Cartan involution of a real semisimple Lie algebra $\mathfrak{g}$ with Killing form $B:\mathfrak{g} \times \mathfrak{g} \to \mathbb{R}$ is a Lie algebra involution $\theta:\mathfrak{g} \to \mathfrak{g}$ so that $(X,Y):=-B(X,\theta(Y))$ is an inner product on $\mathfrak{g}$.
It is claimed that every real semisimple Lie group has a Cartan involution, and that it is unique up to an inner automorphism.
I am looking for a proof of this claim.