Brennan was the key to the operation because the Foreign Intelligence Surveillance Act (FISA) court refused to approve several requests by the FBI to initiate taps on Trump associates and Trump Tower as there was no probable cause to do so but the British and other European intelligence services were legally able to intercept communications linked to American sources. Brennan was able to use his connections with those foreign intelligence agencies, primarily the British GCHQ, to make it look like the concerns about Trump were coming from friendly and allied countries and therefore had to be responded to as part of routine intelligence sharing. As a result, Paul Manafort, Carter Page, Donald Trump Jr., Jared Kushner and Gen. Michael Flynn were all wiretapped. And likely there were others. This all happened during the primaries and after Trump became the GOP nominee.
In other words, to make the wiretaps appear to be legitimate, GCHQ and others were quietly and off-the-record approached by Brennan and associates over their fears of what a Trump presidency might mean. The British responded by initiating wiretaps that were then used by Brennan to justify further investigation of Trump’s associates. It was all neatly done and constituted completely illegal spying on American citizens by the U.S. government. [Source]