Complex sentence: Immediately after the Argentines had seized the Falkland Islands they estzblished an airbase on Pebble Island using the local airstrip on which were based FMA IA 58 Pucará light ground attack aircraft that could have posed a threat to British ground forces once they had landed on East Falkland.
Split into simpler sentences: Immediately after the Argentines had seized the Falkland Islands they established an airbase on Pebble Island using the local airstrip on which were based FMA IA 58 Pucará light ground attack aircraft. These could have posed a threat to British ground forces once they had landed on East Falkland.