An ARP-based MITM attack is achieved when an attacker poisons the ARP cache of two devices with the MAC address of the attacker's network interface card (NIC). Once the ARP caches have been successfully poisoned, each victim device sends all its packets to the attacker when communicating to the other device and puts the attacker in the middle of the communications path between the two victim devices. It allows an attacker to easily monitor all communication between victim devices. The intent is to intercept and view the information being passed between the two victim devices and potentially introduce sessions and traffic between the two victim devices

The analyst is in the detection and analysis phase of the incident response process according to NIST SP800-61. In this phase, events are detected and analyzed to determine whether they constitute incidents that require a response. It involves monitoring security events or data collection, correlation, and analysis of log entries and network flow data, among others. The goal is to identify incidents quickly so that appropriate actions can be taken. References := NIST SP800-61, Computer Security Incident Handling Guide, Section 3.2:
Detection and Analysis