Active shooter, Field command and equipment.
Columbine was the deciding factor in revamping police policy on active shooter scenarios. Before that, the thinking was to wait it out until the high-speed low-drag guys got there. Now, the first responders are to go in and do what they can to end it, then hand off responsibility to SWAT when they show up.
The Waco and Ruby Ridge incidents changed the way command structure is set up. Prior to these incidents, SWAT commanders outranked on scene negotiators. Because of the mishandling of these events, determined by internal after-action reports and not us "conspiracy nuts", negotiators and SWAT must agree before SWAT engages. In other words, Inline 6, the agencies involved think the same as people you are busting down. Egos got folks killed and policy was changed to prevent it from happening again.
The North Hollywood shootout was the media sensation that caused people, citizens who vote for levies and beaurocracts who decide who gets issued what, that more than a handgun may at times be needed to deal with out of the ordinary circumstances. AR's began turning up in cruisers and every patrol car here in Ohio now has a shotgun. This also accelerated the drive toward more "real world" shooting skill in training, not just tiny groups on paper at 25 yards.
The WTC attack of 9/11/01 revealed a dire need for much more reliable and streamlined comms. This has been slow to become a working reality, but revamping and equiping procedures and equipment doesn't come fast or cheap.
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