Sparta's ally attacked an ally of TLR and TLR committed ourselves to their defense. Where one goes, the rest follow. The beauty of a MDAP. Furthermore, Sparta declared war upon two small alliances prior to the pre-empt on MHA therefore rendering any defense obligations to Sparta as optional.
Would also like to point out that when it was decided that a preemptive attack upon MHA was necessary, the mutual aggression clause of the C&G treaty was invoked upon GATO's request and you know that the C&G treaty we share with one another reigns supreme over all others. ODN couldn't refuse even if they tried.