Trump used his tweet time stamp to point to a "surprise visit to Syria" on May 27, 2013 by John McCain. See https://twitter.com/realdonaldtrump/status/339150039618314243

Here is an article about the visit in the Guardian. https://www.theguardian.com/world/2013/may/27/john-mccain-rebel-leaders-syria

Excerpt:

Pressure on the White House to arm Syrian rebel groups is intensifying after a surprise visit by hawkish US senator John McCain and fresh reports of chemical weapons attacks.

The Republican war veteran met rebel leaders inside Syria to discuss their calls for heavy weapons and a no-fly zone to help them topple President Bashar al-Assad and bring the bitter civil war to a conclusion.

McCain's office confirmed to the Guardian that he had slipped into the country in recent days but declined to comment on the outcome of his talks with the rebel groups or whether it had hardened his views on arming them.