WASHINGTON — In the days since President Trump ordered a cruise missile strike against Syria in retaliation for a chemical attack on civilians, his administration has spoken with multiple voices as it seeks to explain its evolving policy. But one voice has not been heard from: that of Mr. Trump himself.

As various officials have described it, the United States will intervene only when chemical weapons are used — or any time innocents are killed. It will push for the ouster of President Bashar al-Assad of Syria — or pursue that only after defeating the Islamic State. America’s national interest in Syria is to fight terrorism. Or to ease the humanitarian crisis there. Or to restore stability.

The latest mixed messages were sent on Monday in both Washington and Europe. Secretary of State Rex W. Tillerson — during a stop in Italy on his way to Moscow for a potentially tense visit, given Russian anger at last week’s missile strike — outlined a dramatically interventionist approach. “We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world,” he said.

Hours later, Sean Spicer, the White House press secretary, said at his daily briefing that Mr. Trump would act against Syria not just if it resorted to chemical weapons, like the sarin nerve agent reportedly used last week, but also when it used conventional munitions. “If you gas a baby, if you put a barrel bomb into innocent people, I think you will see a response from this president,” Mr. Spicer said.