WASHINGTON — President Trump made clear on Wednesday that missiles “will be coming” at Syria at any moment, telegraphing a military operation as he has previously said he would never do. But the real suspense remained — how many missiles, for how long, at what targets and to what end.

The strike that Mr. Trump was preparing as retaliation for a suspected chemical attack carries all sorts of perils that worry military planners and diplomats alike. A fresh intervention in one of the most combustible battlegrounds on the planet — one already crawling with Syrian, Russian, Iranian, American, Turkish and Kurdish forces — could easily bring unintended consequences.

The more expansive the strike, officials and experts said, the greater the risk of accidental casualties that could deepen the conflict with Russia or Iran. Yet a more restrained operation might not inflict enough damage on the government of President Bashar al-Assad to change his calculations.

If Mr. Trump goes beyond missiles and authorizes the use of manned aircraft even from outside Syrian airspace, they face the dangers of a modern air defense system provided by Moscow.