The leader of the protestors actually stated on western media that they needed “bloodshed”, “only then will the Chinese people wake up”, so idk how “peaceful” it was supposed to be. And the army didn’t attack them, in fact the fights only started after both the army and protestors had left the square and a few protestors attacked the anti-riot policemen.
And it’s common practice to mobilize the army anyway, Spain has done every time a territory has tried to declare itself independent. The Tiananmen square protest lasted several days, so obviouly someone had to (peacefully) disperse protestors.