Ukraine

To give some examples from Ukraine, the US has had UAVs, like Global Hawks, regularly orbiting (and broadcasting their position) in Ukraine prior to the invasion, and after, just outside Ukraine. These are intelligence-gathering platforms with fancy radars and sensors. The US is going to be transferring data from those to Ukraine. The US sent out JSTARS aircraft (which were right on the brink of retirement, and were just retired last year), aircraft with airborne radar that can track ground vehicles in an area right before and during the invasion; no doubt they were telling Ukraine what vehicles Russia was moving around.

The US has also had a whole lot of tanker aircraft -- with transponders on -- fly right up to the border and sit right outside of Ukrainian airspace doing laps, then returning to base. It's a pretty safe bet that they aren't going up there to just see the sights; the US Air Force published at least one photo of one taken from a tanker refueling an F-35 that showed a bit of ground. That was geolocated to a point in Poland just outside Ukraine. The F-35s have not had their transponders on, and the one shown in the photo was flying in stealth configuration, with the radar reflectors removed. They can act as intelligence-gathering platforms, and at the minimum they were flying right next to Ukraine, if not over it or in other airspace in the area. And it's probably a pretty good bet that, while they aren't shooting, they've been telling the Ukrainians what they see. And Ukraine may go and shoot at things that they see.

Is that a causus belli? Well, the US doesn't consider it to be one, but over history, I expect that different countries would have taken different views on that matter. I imagine that Russia could decide to treat it as one, but I also imagine that Russia isn't looking to have a direct war with the US, so probably isn't going to bring it up.

Similarly, when Russia had that Su-27 dumping fuel on that Reaper and the Russian pilot dicked up and collided with the Reaper's propeller, bringing it down, was that a causus belli? Well, the actual collision was, no doubt, accidental -- Russia isn't going to risk losing a manned fighter jet to bring down a Reaper -- but dumping fuel on the UAV, something which could hypothetically bring it down, could probably be considered a form of attack. But...the US has decided not to treat that as a causus belli, as the US isn't looking to kick off a direct war with Russia either. Both the Su-27 and the Reaper can carry air-to-air weapons, but Russia wasn't shooting at Reapers -- just dumping fuel on them -- and the US isn't shooting at Russian fighters that get too close to US drones.