His books had told him nothing of the Nome King's Magic Belt, the Country of the Nomes being outside the Land of Oz.

He knew, however, that he was likely to be conquered unless he made a fierce fight, so he spread his wings and rose in the air and flew directly toward Dorothy.

The Wall of Glass had disappeared the instant Ugu became transformed.
Dorothy had meant to command the Belt to transform the magician into a Dove of Peace, but in her excitement she forgot to say more than "dove," and now Ugu was not a Dove of Peace by any means, but rather a spiteful Dove of War.

His size made his sharp beak and claws very dangerous, but Dorothy was not afraid when he came darting toward her with his talons outstretched and his sword-like beak open.

She knew the Magic Belt would protect its wearer from harm.
But the Frogman did not know that fact and became alarmed at the little girl's seeming danger.