<text>  CNNIn an audio message purportedly from an ISIS spokesman, the group announced that a pledge of allegiance from Nigerian-based Boko Haram has been accepted by ISIS leader Abu Bakr al-Baghdadi. The 28-minute message, which cannot be independently authenticated by CNN, was posted online by ISIS supporters. The message says that the caliphate, or Islamic State, has expanded to western Africa and congratulated our jihadi brothers there. The spokesman, Abu Mohammed al Adnani, encourages people to join fighters in Africa if they cannot make it to Iraq or Syria. Boko Harams leader, Abubakar Shekau, announced in an audio message last week that the Islamist terror group was going to ally with ISIS. Jacob Zenn, a terror expert who lives in Nigeria, told CNN on Saturday the alliance would make sense for both groups. Boko Haram will get legitimacy, which will help its recruiting, funding and logistics as it expands, Zenn said. It will also get guidance from ISIS in media warfare and propaganda. Previously Boko Haram was a sort of outcast in the global Jihadi community. Now it is perhaps ISISs biggest affiliate. ISIS gets more international legitimacy as a global caliphate. Boko Haram, whose name translates as Western education is sin, has been waging a yearslong campaign of terror aimed at instituting its extreme version of Sharia law. Boko Harams tactics have intensified in recent years, from battling Nigerian government soldiers to acts disproportionately affecting civilians -- such as raids on villages, mass kidnappings, assassinations, market bombings and attacks on churches and unaffiliated mosques. Much of this violence has taken place in Nigeria. But neighboring countries, such as Cameroon and Chad, have also been hit increasingly hard. What Boko Harams pledge of allegiance to ISIS means . CNNs Paul Cruickshank contributed to this report. 