Specifically, Trump tweeted that
I have instructed the United States Navy to shoot down [sic] and destroy any and all Iranian gunboats if they harass our ships at sea.
From the moment of the tweet, it is reported that the NYMEX West Texas Intermediate (WTI) front-month contract - the benchmark for American crude oil - was trading at about $11.52 per barrel. After 20 minutes, the crude oil increased by 40%, reaching the $16.12 per barrel.
The rise is believed to be the outcome of the investors' optimism of instability in the Strait of Hormuz, with the potential of restricting Persian Gulf oil shipments, thereby reducing supply in a glutted market.
By Monday the world saw the WTI trading in negative territory due to low demand and brimming storage capacity due to the COVID-19 crisis.
Referring to Trump's tweet, Dryad Global, security consulting, commented that the tweet most likely was a response to recent IRGC(N) harassment of the US navy, as well as an attempt to influence events within the oil market.