I would be surprised if a group of contractors funded and directed by American sources but who happen to live in Argentina would be classed as Argentinian, but then I'm not a lawyer and there's lots about the law that surprises me. One example would be Hezbollah - they are based in Lebanon but believed to be funded by Iran, and so what they do is often considered to be done by Iran not by Lebanon.
With the US v Nicaragua point, I've never really known much about what went on with Reagan and the Contras, but from having a look on Wikipedia it seems that:
- If you mean the US attack on Nicaragua, that was based on the US's unilateral opinion rather than an agreed international opinion. As such I wouldn't count that as a widely held agreement on what counts as an act of war
- If you mean the ICJ decision on the aftermath of the US attacks I would agree that it is at least a consensus decision on what acts are considered valid
- From the ICJ decision:
- Nicaragua supplying arms to El Salvador opposition does not constitute an armed attack (and thus I guess would not be an act of war). This seems in line with your suggestion that financing someone in a foreign country doesn't make them your army, the army belongs to the country in which they live not the country which supplies them.
- However it also suggests that the US arming, financing and training the contra groups intervened in the sovereignty of another state, and through various attacks the US was considered to have used force against another state. While not an act of war it seems in that direction which is the other way round to the findings in the Nicaragua v El Salvador point above - although the contras were based in Nicaragua since they were US funded, they are considered to be US?
I guess there are particular circumstances with each of the two above, ie perhaps the US attacks on Nicaragua were much more clearly linked to the US government. Also there's a lot of different actions ("act of war", "armed attack", "intervene in sovereignty", "used force" - what do they all mean?). I have some more reading to do it seems!