Personally, I see this as a very simple situation.

There cannot be both WAR and a PEACE TREATY in effect at the
same time. Shallam stood by the treaty until such time as Tylin
declared War.

At that point, Tylin, a citizen of Ashtan, in effect
publically denounced any treaty. Surely not even Tylin,
as dishonourable as he is, expects the protection of
a treaty he himself does not recognize.
And Shallam cannot be expected to abide by a treaty when a
rogue Ashtanite acts the way Tylin has acted. Ashtan should
act more responsibly, and take note when its citizens declare war
during times of tenuous Peace.

Dr. Taranis, of the Alternative Medicines

ps - These are my personal beliefs and do not necessarily reflect
the opinions of Shallam.

Penned by my hand on the 7th of Lupar, in the year 236 AF.