 Apo Austria yakabudirira mukudzvinyirira kupandukira, Van Merlen akanga ari mumwe waavo vakatizira kuFrance, vachigara muDouai muna December 1790. Musi wa15 Chikunguru 1792, akagadzwa 2nd Lieutenant muBelgian Legion, chikamu cheHondo yekuchamhembe pasi paCharles François Dumouriez. Akarwa paJemappes neNeerwinden, achiva 1st Lieutenant musi wa13 March 1793 uye Captain musi wa11 August gore rimwe chetero. Muhondo yeLincelles, mbabvu dzake mbiri dzakapwanywa nebara rechivhorovhoro. Gore rakatevera, akava mubatsiri waJean-Baptiste Dumonceau, uye akatora chikamu muhondo yeTourcoing. 