None of that makes the US (or any other Ukrainian supplier) a legitimate target in terms of international law. Especially as they're nearly all defensive system

None of that makes the US (or any other Ukrainian supplier) a legitimate target in terms of international law. Especially as they're nearly all defensive systems being used inside Ukrainian territory. We've established this precedent over decades in earlier proxy wars like Vietnam and Afghanistan where the US and USSR armed each others opponents. It certainly doesn't make the supplier an 'effective war participant' as you stated. This was even true when the US and USSR sent actual personnel in as 'advisors' or combat pilots, even though that is direct involvement. It's not a casus belli.