  Let $(M^n,g)$ be simply connected, complete, with non-positive sectional
curvatures, and $\Sigma$ a 2-dimensional closed integral current (or flat chain
mod 2) with compact support in $M$. Let $S$ be an area minimising integral
3-current (resp. flat chain mod 2) such that $\partial S = \Sigma$. We use a
weak mean curvature flow, obtained via elliptic regularisation, starting from
$\Sigma$, to show that S satisfies the optimal Euclidean isoperimetric
inequality: $ 6 \sqrt{\pi}\, \mathbf{M}[S] \leq (\mathbf{M}[\Sigma])^{3/2} $.
We also obtain an optimal estimate in case the sectional curvatures of M are
bounded from above by $-\kappa < 0$ and characterise the case of equality. The
proof follows from an almost monotonicity of a suitable isoperimetric
difference along the approximating flows in one dimension higher and an optimal
estimate for the Willmore energy of a 2-dimensional integral varifold with
first variation summable in $L^2$.
