The early years of Erdoğan's tenure as prime minister saw advances in negotiations for Turkey's membership of the European Union, an economic recovery following a financial crash in 2001 and investments in infrastructure including roads, airports, and a high-speed train network. He also won two successful constitutional referendums in 2007 and 2010. However, his government remained controversial for its close links with Fethullah Gülen and his Gülen Movement (since designated as a terrorist organisation by the Turkish state) with whom the AKP was accused of orchestrating purges against secular bureaucrats and military officers through the "Balyoz" and "Ergenekon" trials. In late 2012, his government began peace negotiations with the Kurdistan Workers Party (PKK) to end the Kurdish–Turkish conflict (1978–present). The ceasefire broke down in 2015, leading to a renewed escalation in conflict. Erdoğan's foreign policy has been described as Neo-Ottoman and has led to the Turkish involvement in the Syrian Civil War, with its focus on preventing the Syrian Democratic Forces from gaining ground on the Syria–Turkey border during the Syrian Civil War.