What would European leaders fear in The Communist Manifesto?

The most elemental answer here is the power of
transformative change.  At its most pure essence, Marx/ Engels' work is one that
advocates for a radical shift in the ownership of the means of production and the
allocation of resources within it.  This means that leaders in both business and
politics were put "on alert" with the messages coming out of the pamphlet.  Marx/ Engels
are stressing the idea that the time for change is at hand and at the specific moment in
the historical dialectic, the material conditions are ripe for change to happen.  This
would cause European leaders to be scared because it is they who would lose power and
have to experience the force of this revolution and change.  It is the Status Quo who
loses the most in the ideas of Marx/ Engels, reason enough for the European leaders to
hold fear in what is being stressed.