Terrorism is considered invisible warfare because terrorists primarily use invisible gases to harm people most nations have ignored threats victims usually have no warning or immediate knowledge of the attacker the impact of terrorism is nearly invisible

the answer is victims usually have no warning or immediate knowledge of the attacker.

Victims usually have no warning or immediate knowledge of the attacker.

Explanation:

Although terrorism has no official definition, it can be expressed as acts of violence usually in public places where civilians gather, to spread a climate of fear through the community in order to achieve political and/or religious goals.

Terrorism victims usually have no warning or immediate knowledge because it helps terrorists create an environment of fear, and to successfully attack, but there also have been made false warnings to create chaos.