Abbott campaigned in opposition and in office to halt the human trafficking trade, and unauthorised maritime arrivals ceased during his term of office under Operation Sovereign Borders. In foreign policy, Australia continued its military engagement in the Middle East, amid the worsening Syrian conflict. In 2015, the Abbott Government agreed to resettle an additional 12,000 refugees from the region. Abbott challenged Russia at the United Nations over the shooting down of Malaysian Flight MH17 in Ukraine. The Government launched the New Colombo Plan to encourage educational exchange with the Indo-Pacific region. Domestically, Abbott campaigned for recognition of Indigenous Australians in the Australian Constitution, flagging a referendum for 2017, and promised a plebiscite on the issue of same-sex marriage.