So right now I am thinking about a shielding system that would be added to a 20 minutes into the future scenario on naval ships. This shielding system would allow objects moving slower than 400 km/h relative through, while stopping all projectiles moving at speeds higher than that. Charged particles and intense electromagnetic radiation would be stopped regardless.
As the shield stops incoming projectiles, the shield would harden like a non-newtonian fluid and shatter the incoming projectile, partially absorbing some of the kinetic energy. Significant pieces of large, heavy projectiles may remain and continue to strike whatever is behind the shield, and almost all of directed energy weapon impacts will be absorbed. Shield efficiency decreases as the shield absorb energy and approach a saturation point, before the shield finally goes down as it overloads. Shields can be "vented" by releasing pent-up energy as easily detectable electromagnetic radiation into the surrounding environment.
What impacts would this have on navies armed with early 21st-century weapons? How do you think weapons will evolve to counter this shielding technology, without handwaving in a special counter-shield device?