Complex sentence: The Western Allies at first provided all resistance organizations with funds and equipment, giving special preference to ELAS, whom they saw as the most reliable partner and a formidable fighting force that would be able to create more problems for the Axis than other resistance movements.
Split into simpler sentences: The Western Allies at first provided all resistance organizations with funds and equipment. However, they gave special preference to ELAS, whom they saw as the most reliable partner and a formidable fighting force that would be able to create more problems for the Axis than other resistance movements.