Two people ultimately deserve credit for such murderous attacks.
Obama and Soleimani.
The former (along with John Kerry) ensured that the Iranians would have plenty of cash (pallets full), to fund such operations, and Soleimani, as commander of the Quds force, was in charge of the terrorist outreach program carried on by Iran.
Soleimani and Iran gave considerable aid and advice to the Houthis.