Complex sentence: Immediately after the Argentines had seized the Falkland Islands they created an airstrip on Pebble Island on which were based Pucará light attack aircraft that could have posed a major 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 created an airstrip on Pebble Island. On it they based Pucará light attack aircraft that could have posed a major threat to British ground forces once they had landed on East Falkland.