Japan's Nikkei dropped 0.4 percent, while the S&P 500 lost 0.55 percent. Trump's move also sent oil prices to their highest levels since late 2014 due to fears of military conflicts in the Middle East. Investors find gold their safe-haven assets.
Though Syria is not a significant oil producer itself, the wider Middle East is the world's most important crude exporter and tension in the region tends to put oil markets on edge.
