Hi, not a historian but I'm from Argentina and could give you some perspective.

If Argetina was going to became an independant state, it had to eliminate the threat of the Spanish in Lima. During the first years, a couple of campaigns began in what is now Bolivia and Paraguay to fence any invading colonial army and to try to push our armies up north. The campaigns first led by Castelli and Belgrano were a failure or mediocre at best, Castelli was even imprisoned because of his failure. One of the problems we had was that we lacked proper training for our troops and our generals, both Castelli and Belgrano were only lawyers. During this initial failed campaigns Argentina wasn't invaded thanks to the guerilla warfare taking place in Bolivia at the time.

San Martin arrived in Buenos Aires in 1812. He had a proper military education and most importantly he had experience fighting along side Spanish troops. Even though he didn't lead many armies in our territory his advice and tactical knowledge was crucial to the revolutionary army. He created the first professional cavalry in the army the "Granaderos a Caballos".

During this time we had our first important victories. First, the threat of the crown controlled Banda Oriental (modern day Uruguay) was eliminated and the newly created navy defeated the Spanish fleet in the Rio de La Plata. San Martin's contributited by repelling a raiding force in San Lorenzo, this was the last attempt from the Spanish to try and raid the east provinces. Secondly, we had our first success in the North. Belgrano led a successful army through the second upper-Peru campaign. Even though he had an under trained and under supplied force he defeated and invading army in Tucuman after evacuating the northern provinces of Jujuy and Salta using Scorched earth tactics.

Even though this campaign was a success, the important objective of invading Lima was not even close to being acheived. After a disastrous third campaign, San Martin took command of the northern army. After realizing that invading through the north was sure defeat he devised another plan to cross the Andes and attack Quito and Lima through Chile. The Argentinian war of independance was only finished after Chile was liberated because it destroyed the Spanished forces that could pose a threat to southern provinces. You have to underdtand as well that the crossing of the Andes is almost a mythical tale, here we see as the South American version of Hanibal crossing the Alps, he did this while having Asthma and cholera.

You too have to understand the history of Argentina to understand why he is seen as the most important person in our history. We have almost no military tradition, and we have been involved in only two meaningful wars. The war of Paraguay, the deadliest war in history relative to population so we are not very proud of that, and a disgraceful and utter defeat to the British during our last military dictatorship. So San Martin story of liberating our country while crossing the Andes, liberating Chile and helping Bolivar in the North to liberate Colombia and Peru makes him up to be like a God o Super Hero.