Intentionally downing drones has typically not been causus belli. Iran downed a US stealth drone intentionally and seemed to have made it out with essentially no consequences.
https://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incid...
https://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incid...
On 9 December 2011, Iran lodged a formal complaint to the United Nations Security Council over the UAV violating its airspace.
Iran's U.N. ambassador stated in the letter that "My government emphasizes that this blatant and unprovoked air violation by the United States government is tantamount to an act of hostility against the Islamic Republic of Iran in clear contravention of international law, in particular, the basic tenets of the United Nations Charter."