China continues making more threats against foreign (especially American) use of international (more than 22 kilometers offshore) waters for their recon ships and aircraft. This has produced some scary incidents over the last two decades and the most recent one was on August 19th where, several hundred kilometers off a major naval base on Hainan Island a Chinese Su-27 jet fighter intercepted an American P-8 maritime patrol and anti-submarine aircraft. The Chinese fighter flew close (less than ten meters) to the U.S. aircraft and made threatening maneuvers that were captured on video. China denied the incident occurred as described, despite the video evidence.
Such encounters are increasingly common, as China seeks to drive American military ships and aircraft out of the South China Sea, which China claims as its sovereign territory. International law does not recognize such claims, but China is determined to prevent its naval training operations from being observed. At the same time China is increasingly active in sending its own recon ships and aircraft to observe what the U.S. and neighboring countries are doing.
Just keeping track of the enemy's electronic devices has long been a major peacetime task, especially since no one knows exactly how everyone’s electronic equipment will interact until there is a sustained period of use. Such use does not occur in peacetime, when the EW equipment is used infrequently for training and testing. All electronic equipment has a unique electronic signature. Even equipment that is not broadcasting will appear a certain way to various sensors like radar or sonar. Thus a critical peacetime function is to determine what these signatures are.
Then there is anti-submarine work, where you want to record what foreign subs sound like and the easiest place to record them is near the ports they are based at, as they go out and return from training. For this reason navies and air forces devote a significant amount of their time tracking other nation’s capabilities.
China also wants to prevent Americans from detecting Chinese subs, especially nuclear missile carrying ones, as they leave the new submarine base on Hainan Island. The Chinese know that during the Cold War American attacks subs routinely tracked Russian ballistic missile subs with orders to sink those subs if the sounds of missile launch were detected. Such a policy applied to Chinese missile boats would negate the usefulness of those subs. Since China does not have many land based ICBMs that can reach the United States, the chances of lots of nukes hitting America increase if those nuclear ballistic missile subs are free to operate. None of the Chinese missile subs has actually made a “combat patrol” yet but that is supposed to change in the near future.