Force majeure excuses performance prevented by events outside a party’s control. It is a creature of the contract, not of the general law, and its reach is whatever the clause says.
The event must be listed or captured. Many clauses enumerate specific events — war, fire, flood, act of government — followed by a catch-all. Whether the catch-all reaches an unlisted event depends on the drafting and on the interpretive principle that a general term following specifics is read in their light.
Prevention, not inconvenience. Most clauses require that performance be prevented or rendered impossible, not merely more expensive. Rising costs, supply disruption and loss of a key customer generally do not qualify unless expressly included.
Causation and mitigation. The event must actually cause the failure, and most clauses require reasonable efforts to overcome it.
Notice. Almost every clause requires prompt written notice, often within a stated number of days, and failure to give it can forfeit the excuse entirely.
Where no clause exists, common-law doctrines of impossibility, impracticability and frustration of purpose may apply — but they are narrower than most parties expect and rarely excuse performance made merely uneconomic.