Current members of the military have been selected and trained differently from their predecessors, and the overall environment, expectations, and responsibilities of the modern military force have also changed dramatically over the past few decades. In prior wars, military service was seen as a necessary and difficult but temporary interruption to civilian life. Soldiers joined or were conscripted, were quickly trained, sent into combat, and then returned to civilian life once hostilities ceased. During these conflicts, the primary risk for stress-related disorders was direct exposure to intense combat. Following Vietnam, however, the era of the Cold War resulted in a more stable military structure. The US military transitioned to an all-volunteer force composed of both active-duty and reserve components [7]. This modern fighting force tends to include more women, more married service members, and generally well-educated individuals who are more socially and politically conservative than their civilian counterparts [8, 9]. It is possible that volunteering for military service may enhance resilience to traumatic events as members are more committed to a military lifestyle and more fully supportive of the military mission . Reports suggest that individuals experience better outcomes following traumatic events if they believe their combat experiences served a higher purpose [10, 11].

The stability of the Cold War era allowed for a more predictable military career for most service members. Personal and family responsibilities could be anticipated and planned for without serious disruption by frequent or extended deployments. Beginning with the Persian Gulf War in 1990–1991 and continuing through current conflicts, however, the nature of military service became more unpredictable. Uncertainty of tour length, limited time spent in garrison (dwell time), and instability of garrison location have all been shown to be stressors associated with modern warfare [12].

Mental Health Advisory Team (MHAT) reports in Iraq and Afghanistan have consistently shown that longer deployments are related to a variety of risk factors and behavioral health indices. The longer a soldier has been in theater, the more likely he or she is to accumulate combat experiences. Multiple deployments are also a risk factor for a variety of well-being indices. NCOs on their second and third (or more) deployments have been shown to have more psychological problems and more use of medications. Multiple deployments also result in a greater intent to divorce and separate. Specifically, those on their second deployment report a significantly higher likelihood of divorce intent than those on their first deployment [13].

In some cases these psychological effects of deployment may be self-limited however, and survey data has demonstrated that mental health problems following a 1-year deployment begin to return to pre-deployment levels after approximately 2 years at home station and essentially reset after 3 years at home [14]. This is important to consider, as the changing nature of war has led to increased frequency of deployment, longer deployments, and shorter “dwell time” in garrison and at home. This dwell time is important, as events that occur during this time may help or hinder the service member in preparing for future or repeated deployments.

The extent to which the nation endorses the military mission and supports the troops can also have an impact on the mental health of service members both serving in combat and after returning from war [15]. Whereas during Vietnam public opinion had a profound negative impact on returning soldiers and veterans, more recent conflicts have seen greater public support both for the military mission and for the troops themselves – although with continued variation over time.

A small portion of society is executing today’s wars, and much of the population remains largely unengaged, which may contribute to feelings of isolation among soldiers and veterans. While this may play a smaller role than it did post-Vietnam, it still may represent a risk factor for certain groups such as reservists and National Guard who return to homes, jobs, and communities without the support of military colleagues [2].

With the evolution of modern warfare and the stressors surrounding combat, both the traumatic events that service members are exposed to and the ways in which they perceive those events have changed over time. Research has shown that life events can have a direct impact on individual’s health and well-being [16–19]. Understanding the interplay between these exposure events and individual responses is important for developing appropriate training and intervention strategies to mitigate the potential negative effects associated with combat and war.

The emotional and behavioral responses after combat and disaster occur in four phases. The first phase consists of strong emotions, including feelings of disbelief, numbness, fear, and confusion. These are often normal emotional responses to an extraordinary event. The second phase may last from weeks to months and involves efforts at adaptation to the new environment. Intrusive memories in the form of flashbacks or nightmares as well as hyperarousal may emerge during this phase. Somatic symptoms such as insomnia, fatigue, dizziness, headaches, and nausea are also common. The third phase is marked by feelings of disappointment and resentment if hopes for restoration of the pre-trauma emotional and physical environment are not met. The final reconstruction phase may last for years as survivors attempt to rebuild their lives and social and occupational identities. Individuals progress through these phases at different rates and may develop symptoms at different times in response to the same event [1].

Different people will react differently in a combat environment or when exposed to a traumatic event such as a man-made or natural disaster. All people will experience some level of emotional distress in such an environment. A smaller proportion of the population will go on to have behavioral changes that may be dysfunctional, and only a much smaller subset of those exposed will develop a psychological disorder. These reactions are illustrated in Fig. 1.1.

Distress-related symptoms are universally experienced during a disaster or in the midst of combat. Health providers must be careful not to reinforce a view that the symptoms constitute a disease, as they represent normal responses that are common and usually transient. In the context of ongoing battle, the perception that one is ill can itself lead to impaired functioning and may increase long-term disability [1].

Exposure to traumatic events during combat may result in well-defined psychiatric illnesses in certain individuals. Post-traumatic stress disorder (PTSD) is one such condition that has received a great deal of attention in combat and disaster situations. The Diagnostic and Statistical Manual Fifth Edition (DSM-5) defines PTSD as an exposure to a threat to physical integrity with a resulting emotional response involving helplessness, horror, or fear. Associated symptoms include re-experiencing phenomena such as flashbacks and nightmares; hyperarousal including insomnia, hypervigilance, and increased startle; and emotional numbing or avoidance behavior resulting in social or occupational dysfunction. If these symptoms are transient (less than 1month), they are classified as acute stress disorder, and after 1 month the diagnosis of PTSD is made. Other disorders that may result from exposure to traumatic events are illustrated in Table 1.2.