Complex sentence: Politically it was both a symbol of U.S.-Australian relations, and highly controversial; this was mainly due to fears that the site could prompt a nuclear attack on Australian soil, and antipathy towards the US alliance amongst the Australian political left.
Split into simpler sentences: Politically it was both a symbol of U.S.-Australian relations, and highly controversial in Australia. This was mainly due to fears that the site could prompt a nuclear attack on Australian soil, and antipathy towards the US alliance amongst the Australian political left.