Don't mean to be 'that' guy but this is what the Neo-cons in the Trumps cabinet were waiting for (in my opinion of course). Those incremental troop increases and ramped up asset deployments weren't for "show". Funny thing Iran simply isn't afraid this time around and will be striking back with serious intent to harm. What ensues in the days following will be monumental for the region and the US.
Both Iran and the US are in positions where neither side can really back down. If the U.S. were to let this slide (which we can be damn sure they wont), then Trump and the US military will be the laughing stock of the world. It should be noted that this downing of the MQ-4C Triton, being one of the US most expensive assets, is different from a reaper or, RQ-170 or Scan Eagle. The Triton is a big ticket strategic asset held in high value, there isn't that many of them (depends on your definition of many) but they cost a lot of money to produce.
Iran does indeed seem to be prepared to take on what the U.S. has to throw at them. Boldness such as this doesn't come unwarranted.