"[President Barack] Obama must now start focusing on OUR COUNTRY, jobs, healthcare and all of our many problems," he tweeted. "Forget Syria and make America great again!"
Two years ago Mr Trump built a winning presidential campaign around this very theme, sharply criticising Democrats and some fellow Republicans for what he viewed as their overly interventionist foreign policies.
Just a week ago Secretary of State Rex Tillerson appeared to be following this new script when he downplayed calls for Syrian President Bashar al-Assad's removal, saying his status "will be decided by the Syrian people".
Then on Thursday night, two days after another chemical weapons attack, President Donald Trump ordered a guided missile strike on a Syrian government airfield.
The move marks a dramatic turnaround from Mr Trump's prior rhetoric and the expectations of how he would conduct his foreign policy.
In his remarks following the missile strike, the president explained that it was in the "vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons".
He even called on "civilised nations" to end the "bloodshed and slaughter" of the Syrian civil war and insisted that "peace and harmony" will prevail.
The man who was thought to be a neo-isolationist now, just months into his presidency, is projecting US military force abroad to enforce international norms and punish human rights abuses.
What changed? It seems clear that the graphic images of dead and dying Syrian civilians - including "beautiful babies", in Mr Trump's words - had a dramatic effect on the president's disposition.
With a few notable exceptions, the strike is being praised by Washington politicians on the left and on the right. In the coming days, however, some serious questions will be posed.
If the president's foreign policy outlook can shift so dramatically in just a few days, if not hours, will allies and adversaries interpret this as a sign of flexibility or incoherence?
A cruise missile strike is a low-risk form of military action, but it also is of limited effectiveness.
A White House official described this as a warning shot across Mr Assad's bow.
If the Syrian president continues to use chemical weapons or conduct conventional attacks that result in significant civilian casualties, will the US escalate its military intervention or back down and risk appearing weak?
When Mr Obama contemplated using force against the Syrian government, he decided that he would need congressional authorisation to do so. Will Mr Trump now seek approval from the lawmakers, on both sides of the aisle, who were reluctant to give the Democratic president such approval?
Thursday night was Mr Trump's first significant foreign policy challenge, and it appears to have dramatically changed his outlook, his rhetoric and his resulting actions. The candidate who constantly spoke of putting America first ended his brief remarks on Thursday night by calling for God's blessings not just for his nation but for the "entire world".
This strike - and this change of attitude - may simply be a one-off event. Or, perhaps, an unlikely globalist has been born.
Four years ago, after the Syrian government launched a brutal chemical attack on its own civilians, Donald Trump warned anyone who would listen that the US should refrain from launching retaliatory military strikes.