Britain, which has taken the lead in developing EMP (ElectroMagnetic Pulse) devices, is testing a new EMP warhead with the United States. This weapon is intended for use in cruise missiles or UAVs, for attacks on enemy command and control centers. Some of these operations are in bunkers that are well protected from bombs. But an EMP device could at least damage some of the electronics used in those locations. An EMP warhead would also be useful when the target is surrounded by civilians, an increasingly common tactic with some governments, who know that Western governments are more reluctant to kill non-military personnel.
EMP was discovered when nuclear weapons were being tested. The nuclear explosion also created an EMP, that damaged or destroyed many electronic components. As electronics got smaller over the last few decades, they became more vulnerable to EMP. You can build EMP shielding into electronic components, but this increases size, weight and cost. These are also usually custom jobs, not available for all components, or to all customers. Because of this, several countries have been researching ways to generate an EMP without a nuclear weapon. Britain has had the most success, and has successfully tested their device several times in the laboratory. But the new tests will be more realistic. Most details of the British device, and the tests, is kept secret (lest potential targets see ways to better protect themselves.)