As armies struggled to catch up to the new technologies, the mere act of mobilizing your armed forces was enough to start an armed conflict, because once your mechanized troops were massed somewhere, they could steamroll through a neighboring country before there was any chance to react. And so, countries began taking the mobilization itself as an act or war. This had catastrophic effect in 1914 when Germany demanded that Russia demobilize its troops (sent in support of Serbia) within 12 hours. This was a very short timeframe for the era, leaving no time for negotiation or airing of grievances, but it was a necessary one due to the newfound speed potential of mobilized armies. The result was World War 1.
ICBMs rekindled this problem, which is why people were so shit scared during the 60s, 70s, and 80s. Anything that allows you to neutralize your enemy's defenses before they have a chance to react increases the chance of their preemptively starting a conflict.
Unfortunately, the genie is already out of the bottle on hypersonics, and we'll just have to hope that we can reach a stable equilibrium before certain countries feel mortally threatened enough to start a huge war.