sich widersetzen
to resist
To actively resist a person, an order, a law, or an instruction and refuse to obey.
Er widersetzte sich der Festnahme durch die Polizei.
He resisted arrest by the police.

to resist / oppose
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to resist
To actively resist a person, an order, a law, or an instruction and refuse to obey.
Er widersetzte sich der Festnahme durch die Polizei.
He resisted arrest by the police.

The verb 'sich widersetzen' (to resist) is always used with the dative. So you resist *something* or *someone*.
Die Mitarbeiter widersetzten sich der neuen Regelung.
The employees resisted the new regulation.
This verb is reflexive, meaning it is always used with a reflexive pronoun (sich, mich, dich, etc.). The pronoun must match the subject of the sentence.
Ich widersetze mich diesem unsinnigen Plan.
I am resisting this absurd plan.
The word consists of the prefix 'wider-' (against) and the verb 'setzen' (to set/put). So, figuratively, you 'set' yourself 'against' an instruction or a person.
Mutig widersetzte sich die kleine Gruppe der großen Armee.
The small group bravely resisted the large army.
to contradict
'widersprechen' and 'sich widersetzen' sound similar and both have to do with resistance or opposition. But the difference is important. 'widersprechen' happens mainly with words: you say that you do not agree. 'sich widersetzen' is stronger and more active. It means that you oppose an instruction, an order, a law, or a person and refuse to obey. Learners may confuse the words because both express opposition. Remember: you 'widersprechen' an opinion; you 'widersetzt' yourself to an order or instruction.

Die Mitarbeiter widersetzen sich der Anweisung, weil sie sie für gefährlich halten.
The employees resist the instruction because they consider it dangerous.
Nach dem Gespräch widerspricht Frau Hofmann der Entscheidung ihrer Chefin.
After the conversation, Ms. Hofmann contradicts her boss's decision.