In the first half of 1943 two Axis offensives came close to defeating the Partisans.  They are known by their German code names Fall Weiss  and Fall Schwarz , as the Battle of Neretva and the Battle of Sutjeska after the rivers in the areas they were fought, or the Fourth and Fifth Enemy Offensive, respectively, according to former Yugoslav historiography. On 7 January 1943, the Bulgarian 1st Army also occupied south-west Serbia. Savage pacification measures reduced Partisan activity appreciably. Bulgarian infantry divisions participated in the Fifth anti-Partisan Offensive as a blocking force of the Partisan escape-route from Montenegro into Serbia and in the Sixth anti-Partisan Offensive in Eastern Bosnia. Negotiations between Germans and Partisans started on 11 March 1943 in Gornji Vakuf, Bosnia. Tito's key officers Vladimir Velebit, Koča Popović and Milovan Đilas brought three proposals, first about an exchange of prisoners, second about the implementation of international law on treatment of prisoners and third about political questions.  The delegation expressed concerns about the Italian involvement in supplying the Chetnik army and stated that the National Liberation Movement is an independent movement, with no aid from the Soviet Union or the UK. Somewhat later, Đilas and Velebit were brought to Zagreb to continue the negotiations. In the Fourth Enemy Offensive, also known as the Battle of the Neretva or Fall Weiss , Axis forces pushed Partisan troops to retreat from western Bosnia to northern Herzegovina, culminating in the Partisan retreat over the Neretva river. It took place from January to April, 1943.. How many months did the Fourth Enemy Offensive take place over?