If it rains, the garden party will probably be thrown into chaos.

Literal

By-chance rain [subject-が] fell-[conditional-たら], garden-party [subject-が] will-fall-into-disarray [probably-だろう].

混乱する ('to fall into confusion/chaos') — a formal Sino-Japanese verb for disorder or disruption. Different from simple 中止 (cancellation) — 混乱 suggests active chaos rather than clean suspension.