Complex sentence: During the 1950s, the Department of Defense initially proposed lithium/fluorine as ballistic missile propellants, but an accident at a chemical works in 1954 where a cloud of fluorine was released into the atmosphere convinced them to instead use LOX/RP-1.
Split into simpler sentences: During the 1950s, the Department of Defense initially proposed lithium/fluorine as ballistic missile propellants. A 1954 accident at a chemical works where a cloud of fluorine was released into the atmosphere convinced them to instead use LOX/RP-1.