Not many do polemology but, by the way, a secret, small-scale incursion of a state's capital, with perhaps two dozen agents entering its borders with the intent to use force to topple the government, is secret armed conflict from the time of the agents' entry into the country or from the time their act commences from within it, in my opinion.

In part or in full, #CIA-backed agents parachuting into Bulgaria for Project QKSTAIR/BGCONVOY during 1949-1956, contrary to Article 2, subsection 4 of the UN Charter, is an example for the case of incursion into the country with warlike intent. 

(The other case, of a purely internal act to topple the government, would be treated differently from a legal standpoint.)