This is itself normally a police issue. And the group that would normally deal with the violence is also the police. That is part of the way a civil society operates once it has nuetral institutions. The army going so far as enforcing their request would have compromised their non partisanship in politics and so would have suppressing the freedom of assembly of the other group. The military is supposedly there across transfer of power from government to government and play little if any role in that transfer of power. In this case the limitations are not forcing the regime out of power and not suppressing the demonstrators protest. Within that asking protestors to leave was one extreme and tolerating people coming to the square to use force against others is the other. One suspects, that there will greater vetting of entry to the square in future (and even allocating different areas for the two "unarmed" groups).