If you describe a situation, relationship, or someone's mood as brittle, you mean that it is unstable, and may easily change. These incidents suggest the peace in Northern Ireland is still brittle = fragile
(of metal or glass) not annealed and consequently easily cracked or fractured lacking warmth and generosity of spirit; "a brittle and calculating woman
Having hardness and rigidity but little tensile strength A brittle material breaks readily under excessive load with a comparatively smooth fracture, as glass does
caramelized sugar cooled in thin sheets having little elasticity; hence easily cracked or fractured or snapped; "brittle bones"; "glass is brittle"; "`brickle' and `brickly' are dialectal"