From 2000 to 2006, Hezbollah made a strategic choice to build a massive amount of military infrastructure in southern Lebanon south of the Litani river. In addition, Hezbollah enormously increased the quantity and the quality of their weaponry, acquiring advanced anti-tank missiles like the AT-14 Kornet. Starting just before the Israeli withdrawal, Hezbollah built significant military infrastructure in southern Lebanon. The IDF and independent observers knew that Hezbollah had built some infrastructure, but the scale was unanticipated. The interwar period was marked by limited, sporadic fighting and few casualties on either side: between 2001 and 2004, Hezbollah launched just 16 attacks. Hezbollah continued to study Israel and adopt to lessons learned, and the group spent a large amount of effort gathering information about Israel. Changing its strategy of high mobility, Hezbollah instead became relatively fixed with pre-built bunkers, stockpiles, and fighting positions. During this time, Hezbollah also acquired its first UAVs from Iran and sent militants to Iraq to train "Special Groups" to wage guerrilla warfare against the Americans.