From Washington Post:
As Chinese security forces blanketed Tibet and other Tibetan-inhabited areas over the past month, the regular army remained discreetly in the background, under orders to let police take the lead in suppressing the unrest that exploded in Lhasa and quickly spread to adjoining provinces.
The backstage role played by the People’s Liberation Army marked a sharp change from China’s last big protests, in 1989. During that crisis, PLA troops using tanks and automatic weapons moved in to quell rioting in Tibet and crush pro-democracy demonstrations in Beijing’s Tiananmen Square, killing hundreds, perhaps thousands, of unarmed civilians.
The decision to minimize military intervention this time suggested that the Communist Party leadership did not consider the Tibet crisis to be as serious as the 1989 protests, when China’s entire ruling system was thought to be under challenge. Analysts here also saw the recent lower-key response as reflecting official concern over the reaction abroad if the army were to deploy massively against internal unrest as China prepares to host the Beijing Olympics in August.