She gave herself time to gather forces (especially the Golden Company minus elephants), she prepared herself for the siege, fortifying Kings Landing and preparing to use people as meat shield. She waited out the crisis, which removed around half of her enemies armies. Then she had Euron ambush Daenerys fleet, which resulted in more losses, mostly in a dragon, ships and probably the remains of Daenerys sanity.
By following through with the promise she could have had her army mixed into the conflict, and end up in a way less fortified position - with smaller army, surrounded by enemies she doesn't believe to keep their words. Seems she did just fine. Also, I don't think she truly believed in the scope of the whole ordeal. Probably right, since all it took to remove the whole "zombie apocalypse" was to poke 1 particular zombie with Valyrian steel, or potentially dragonglass.