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Has ISIS become the leader of the global jihadi movement?

As militants with the Islamic State of Iraq and Syria (ISIS) continue their bloody rampage across the Middle East, their fight has drawn the attention - and in some cases, the allegiance - of like-minded extremist groups across the globe.

Boko Haram, the Nigerian group responsible for a number of atrocities, including last year's brazen kidnapping of nearly 300 schoolgirls, became the latest addition to this growing jihadi coalition over the weekend, when its leader pledged his group's allegiance to ISIS in an online video.

"We announce our allegiance to the Caliph of the Muslims... and will hear and obey in times of difficulty and prosperity, in hardship and ease, and to endure being discriminated against," Abubakur Shakau said in a tweet attached to the video, according to the Associated Press.

The new partnership, at least in the near term, won't necessarily increase the potency of ISIS as it continues fighting in Iraq and Syria, according to counterterrorism experts. But it could help expand the group's reach into Africa, and perhaps most importantly, it could help ISIS solidify its current position as the reigning Islamic extremist group.

"This actually does make ISIS stronger, at least ideologically," explained CBS News Senior National Security Analyst Juan Zarate. "You're not necessarily going to see a flood of Boko Haram operatives floating to Syria to help ISIS fight. That said, what this does is it gives wind in the sails of the notion that the Islamic State is really the vanguard of this global jihadi movement."

Boko Haram's declaration of allegiance follows similar decisions by extremist groups in Uzbekistan, Libya, the Sinai Peninsula, and even the Philippines, who've all aligned themselves with ISIS in recent months.

"You have the reach of the Islamic State all the way from the heart of the Middle East to West Africa," Zarate said. "You now have a wide swath of groups around a range of geographies that are pledging at least support, if not allegiance to the Islamic State. And Boko Haram, which holds territory, is vicious and quite potent, has now added their name to the mix."

Richard Downie, the deputy director of the Africa Program at the Center for Strategic and Internationa Studies, said Boko Haram's pledge of allegiance underscores the degree to which ISIS has eclipsed al Qaeda as the tip of the jihadi spear.

"ISIS has become the industry leader in the last year or so," Downie told CBS News. "When you look at Boko Haram's statements, for example, in the past year or two, there was a period when they had closer links with al Qaeda, particularly AQIM, the African affiliate of al Qaeda. It seems that they've now firmly positioned themselves in the camp of ISIS instead."

Beyond the ideological considerations, the partnership could prove ultimately prove beneficial for both groups in more instrumental ways.

"What you have here is a symbiotic relationship: both groups help each other, at a minimum with propaganda, and potentially in the long term with men, materiel, and resources," Zarate said. "Both [are] now committed to the same cause of the establishment of the Islamic caliphate."

Downie noted that Boko Haram's pledge of allegiance brings a fourth extremist group on the African continent under the banner of ISIS - a development that could spell danger if those groups begin coordinating their efforts.

"There's an Egyptian group, there's a Libyan group - Ansar al-Sharia - and there's an Algerian group as well. One of the concerns is the linkages between these groups, their ability to work together, cooperate, trade fighters and tactics," he said. "With all four of these groups having pledged allegiance to the same master, we could see attempts to coordinate their activities."

Downie also argued Boko Haram could have more immediate goals in aligning itself with ISIS, noting that the group is increasingly on defense in and around Nigeria.

"You could interpret this message almost as a cry for help, for extra resources, whether financial or personnel," he explained. "It comes at a time when the group's under a lot of pressure. Finally, the Nigerian military, in collaboration with neighboring countries, is really putting the squeeze on the group. It's lost a lot of territory in the last two or three weeks, being pushed back from the three states that it controls part of."

As its network of global affiliates grows, ISIS too finds itself under increasing assault at home -- particularly in Iraq, where a combination of U.S.-led airstrikes and local ground forces is making apparent progress in retaking territory seized by militants.

ISIS "is coming under real pressure, there's no question about that," Zarate explained. "The air assault, the offensives coming from the Kurds, not just in Iraq but also Syria, as well as some of the activity that we've seen to cut off their resources - for example, their supply of oil and their ability to take it to market - all of that is being squeezed."

The Iraqi military has succeeded in retaking large parts of Tikrit, a large city in the heart of Iraq's Sunni Triangle that fell under ISIS control last summer. Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told Congress on Wednesday that there's "no doubt" ISIS fighters will be ousted from Tikrit eventually. General Saad Maan of Iraq's Interior Ministry told CBS News correspondent Holly Williams on Tuesday that victory in Tikrit would be secured within days.

That battle is seen as a precursor to the eventual fight for Mosul, Iraq's second-largest city, which has served as a regional hub for ISIS as it attempts to consolidate its grip on the region.

ISIS is reportedly showing signs of strain as it finds itself on defense, with internal fissures arising between the group's domestic members - largely Iraqi Sunnis - and the foreign fighters who have rallied to its banner. But Zarate warned that the West should not assume internal divisions will handicap ISIS in any significant way.

"I don't think we should count on the fact that the Islamic State will implode, because they've got a very steep and real hierarchy and real discipline, internally," he said.