declaring an emergency also gives all kinds of cover for troops to set the stage for doing what they probably came there to do:
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secure the area so operation goes smoothly
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seize evidence
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prevent destruction of evidence (sabotage)
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prevent exfiltration of evidence (e.g., Sandy Berger)
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make arrests
worth considering that most of the swamp consists of mid-level bureaucrats and operatives that we've never heard of or seen.
Those barricades keep antifa out, and evidence in.