  We give a very simple construction of the string 2-group as a strict
Fr\'echet Lie 2-group. The corresponding crossed module is defined using the
conjugation action of the loop group on its central extension, which
drastically simplifies several constructions previously given in the
literature. More generally, we construct strict 2-group extensions for a Lie
group from a central extension of its based loop group, under the assumption
that this central extension is disjoint commutative. We show in particular that
this condition is automatic in the case that the Lie group is semisimple and
simply connected.
