What the title says.
The Hague treaty was signed by the USA in the name of William H. Taft by J. Choate, H. Porter, U. Rose, and a few others, so one may consider it binding.
Trump reportedly (NY Times and Trump on Twitter) cancelled a military attack against several targets in Iran dubbed as "retribution" merely a few minutes before the beginning of hostilies earlier today. Reportedly, the attack had been authorized by the president, and "weapons had been loaded".
To my knowledge, no declaration of war giving reasons or ultimatum with conditional declaration of war as per Article 1 of the Hague traty puts it was communicated. Which makes the beginning of hostilities a strict violation of said article.
Hostilities, this certainly includes espionage drones flying through souvereign airspace, and -- there's little doubt -- loading up guns and running (even though aborting early) an attack on people and material on foreign ground.
Given the wording "minutes", an attack was obviously not an option or consideration, but it was already happening (only just, no shots were fired). After all, you have to identify targets, make a decision, move troops and material, have to get them dressed up and ready etc, etc. Once you're "minutes" before, you have long done all that.
So, I'd say that very much qualifies as "hostilities"?