The focus of attention can be either unitary or divided and can transition from unitary to divided while performing a task. In Experiment 1, we investigated whether alerting hastens the transition from unitary to divided attention. To this end, we employed a dual-RSVP-stream Attentional Blink task (AB; impaired perception of the second of two rapidly sequential targets) with two pairs of letter targets (T1-pair and T2-pair). One component of the AB known as Lag-1 sparing (unimpaired perception of the T2-pair when it is presented directly after the T1-pair) occurs only when the T2-pair falls in an attended location. When the T2-pair falls in an unattended location, the converse pattern occurs (Lag-1 deficit). Accordingly, we used the incidence of Lag-1 sparing/deficit to index whether a location was attended or unattended. We found that presenting a brief brightening flash of the screen (alerting) just before the T1-pair hastened the transition from the initial unitary focus to a divided focus. In Experiment 2, we pitted the hastening account against an alternative hypothesis that the flash triggers phasic activation of the Locus Coeruleus-norepinephrine neuromodulatory system, thus resetting the underlying neural networks that mediate the distribution of attention, triggering a switch from unitary to divided attention. The results of Experiment 2 were incompatible with the hastening account, but consistent with the network-reset account.