I think it's clear that they all died - the soldiers and the Leviathans. In my last viewing, after seeing one of the Leviathans landing belly-up on a building, I ask myself, how will the government get that thing down?
I betcha the US government is going to have a field day with the knowledge they will acquire from this wreckage. They will new weapons, and they will have knowledge of advanced bio-mechanics. I wonder if we will see any of this addressed in the later films.
I think the Chitauri were some of the worst shooters in any sci-fi/fantasy. A Chitauri could enter an office, fire three shoots, and miss a frightened office drone. Even I, who wasn't trained in the military, know that you don't shoot at a moving target, you shoot at where you anticipate them to be. I saw this in the movie when Hawkeye anticipated where a Chariot would be, and nailed it with an explosive arrow. As the Chitauri were such poor soldiers, and they behaved like that animal Thor mentioned that is repulsive and tramples everything in its path, I have some questions:
1.) What was the purpose of the Chitauri? Once they got into a fight with any professional or contractor army, their days were numbered. If they were to frighten the civilian populace, the Other's plan would have backfired. Killing civilians doesn't make them scared; it makes them mad and mobilizes them to fight against the intruders.
2.) Why was SHIELD's 'overlords' so bent on nuking New York City? The state government of New York was already sending in National Guard units to fight the intruders, and, eventually, the United States government would commit forces to the battle. (This would imply the battle had been raging for an hour, at least.) The 'overlords' reaction seemed odd to me, especially when it was clear that the intruders were contained to a 3-block radius around the portal. It becomes even more interesting when it is revealed at the end that a nuke could destroy the command center, and that the portal could be shut down by a specter being jammed into the portal generator.
I was amused by Senator Boynton demanding that the Avengers be held accountable for the damage done to New York. When I saw that, my thoughts went to Ghostbusters II. In that film, it was revealed that the Ghostbusters were sued for damages done to New York City, and had to disband.