Complex sentence: Experience in earlier campaigns had shown that it was preferable to completely unload partially loaded ships than to partially unload completely loaded ships, as the supplies landed would be properly balanced, and there would be no shortages of certain critical items.
Split into simpler sentences: Experience in earlier campaigns had shown that it was preferable to completely unload partially loaded ships than to partially unload completely loaded ships. The supplies landed would then be properly balanced, and there would be no shortages of certain critical items.