SYN: Onset, attack, aggression, onslaught.
ANT: Defense, resistance, repulsion, retaliation, undermining, sapping, stratagem.
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KEY: Assay.
SYN: Test, endeavor, prove, try.
ANT: Survey, conjecture, guess, scan.
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KEY: Assemble.
SYN: Gather, collect, congregate, amass, muster, call together, convoke, convene.
ANT: Scatter, disperse, go away, dismiss.