The elusive figure known only as Wario seems to have had contact with the “Mario brothers” (a nickname for the two founders of a mercenary company, Mario and Luigi, who like to wear matching fatigues on the clock), but otherwise seems uninvolved with them.
We know from whistleblower leaks that Wario does not usually live in the Mushroom Kingdom, but he did make an appearance in the Second Land Invasion (also referred to as Mario Land 2) to try and stop Mario from kidnapping Princess Daisy in Sarasaland (a bid by the Mushroom royal family to secure oil and coastal access to the Red sea, which succeeded).
Was Wario a mercenary hired by the Sarasaland (now deposed) royal family? An external military commander hired to train recruits? He seems driven by material gain and it is unlikely he would have fought for the Sarasaland royal family without monetary incentive.
The investigation is ongoing.