Answer:
San Martín led a rebel army across the _ANDES_ mountains, from Argentina to __CHILE_.
Step-by-step explanation:
San Martin led an army he assembled in Argentina across the Andes to go in Chile and face the Spanish army and liberate Chile from the colonial ruling.
Crossing the Andes was a great challenge... first because of the high mountains present there and the limited number of ways to cross between the mountains. Then, he had to make it so his soldiers would be to be safe once the crossing done.