 Ramba uchiverenga ndima ino - Muna 1787 akagamuchirwa muIrish Brigade yeuto reSpain, uye akabatana neuto rairwisana muCeuta kuchamhembe kweAfrica, pasi paLieutenant Colonel Luis Urbina, uye akasimudzirwa kuva Second Lieutenant. Muna 1791 Mackenna akadzokera kuzvidzidzo zvake muBarcelona uye akashanda somubatsiri wevarwi vakanyoreswa muEurope. Gore rakatevera racho akasimudzirwa kuva Lieutenant muRoyal Regiment of Engineers. Muhondo yePyreneesmukurwisana navaFrench, Juan Mackenna akarwa muRosselló pasi paGeneral Ricardos uye ikoko akasangana nomusununguri womunguva yemberi weArgentina, José de San Martín. Nokuda kwezvaakaita pakudzivirira Plaza de Rozas, akasimudzirwa kuva kaputeni muna 1795. 