The new military policy of hemisphere defense enunciated in 1938 was a reaction to the increasing offensive potentiality of military aircraft. The continental United States could not at that time be invaded or seriously threatened unless the attack was backed by land-based air power. Hostile airplanes in significant numbers could not then be projected across the oceans, but they could be launched from bases established in other parts of the Western Hemisphere within striking distance of the United States and its possessions. The basic mission of the armed forces under the hemisphere defense policy was therefore to prevent the establishment of hostile air bases in the Western Hemisphere. Since eastern Brazil seemed to offer the most likely initial location for such bases, it became the focus of Army plans and actions for hemisphere defense.

In the early summer of 1940 the quick defeat of France and seemingly precarious position of Great Britain created an emergency that required much closer military ties between the United States and the other New World nations. Military collaboration with Canada evolved thereafter with a minimum of friction into a close partnership. Since the Latin American nations at that time had almost no military strength by modern standards, the United States had to assume almost complete responsibility for defending them against external attack. At the same time it had to avoid any infringement-real or imagined on Latin American sovereignty. Aided by the trust inspired by the "Good Neighbor" policy of preceding years, the United States persuaded most of these nations to accept its pledges of armed support, and from June 1940 onward it entered into military collaboration with them to an unprecedented degree. This collaboration included a military association with Mexico almost as close as that with Canada-in marked contrast with the hostile attitude of Mexico toward the United States during World War I. The groundwork of successful collaboration with the other New World nations allowed the United States Army to begin to deploy its offensive strength overseas almost immediately after the Japanese attack.

The opening chapters of the present volume (I-VII) are an introduction to this story and to a sequel volume, Guarding the United States and Its Outposts. These chapters trace the evolution of the policy of hemisphere defense in the three years before Pearl Harbor, the gradual merging of hemisphere defense into the broader national policy of opposing German and Japanese aggression by all-out aid to peoples fighting the Axis, and the quick transition to offensive plans and preparations in December 1941. They describe the crises that faced or appeared to face the United States in May 1940 and again in May 1941 (Chs. II, V), the nature of the German and Japanese threats to the Western Hemisphere (Ch. III), and the preoccupation of the United States with the perilous situation in the Atlantic and Europe that blinded the nation to the menace of Japan in the Pacific until the very eve of open conflict. The next three chapters (VIII-X) relate the general plans and measures of the United States for defending Latin America against attack from overseas and for collaborating with the other American republics toward that end. The chapters which follow describe the specific steps in collaboration with Brazil (XI-XII), Mexico (XIII), and Canada (XIV-XV). The concluding chapter (XVI) is a summary and interpretation.

Key topics:
1. The background and development of war plans for defending the Western Hemisphere (Chs. I-II, IV-VII).
2. The relationship of these hemisphere defense plans to broader war plans (Chs. II, IV-VII).
3. The transition in national military policy from hemisphere to world defense against aggression (Chs. I-VII).
4. The nature and extent of German and Japanese threats to the American continents (Chs. III, V-VII).
5. Military negotiations and relationships of the United States with the other American nations before and during World War II (Ch. VIII).
6. The supply of arms to Latin American nations (Chs. IX, XI-XIII).
7. Preparations for Army air operations in the Latin American area (Ch. X).
8. The establishment and operations of U.S. Army forces in Brazil (Chs. XI, XII).
9. Military cooperation and collaboration between the United States and Mexico (Ch. XIII).
10. Military cooperation and collaboration between the United States and Canada (Chs. XIV-XV; treated more fully in Military Relations Between the United States and Canada: 1939-1945).
11. U.S. Army operations in northern Canada during World War II (Ch. XV).
12. Army-Navy joint action and relationships in support of hemisphere defense clans and measures (Ch. I).
13. The interplay between military and political objectives in planning and executing hemisphere defense measures (Ch. I).
14. The organization and strength of the United States Army, 1939-41 (Chs. I-II, VI).
15. The military weakness of the United States during a period of rapid mobilization (Chs. II, IV-VI).
16. Army plans for emergency expeditionary forces (Chs. I, II, IV-VII).
17. American entry into the Battle of the Atlantic (Chs. I, II, IV-VI).
18. American military policy and plans for action toward European possessions under threat of hostile control (Chs. I, II, IV-VII).
19. The Destroyer-Base Agreement of 1940 (Ch. II).
20. Plans for military action in the Azores, Iceland, and other Atlantic areas during 1940 and 1941 (Chs. IV-VII).

This volume continues the discussion begun in The Framework of Hemisphere Defense by describing how the Army contributed to the security of the national bastion. The discussion naturally divides itself into three sections. The first describes organization of Army forces for protection of the continental United States before and during the war, including elaboration of harbor and air defenses; the Army's role in civilian defense; and protection against subversion after Pearl Harbor, including a detailed account of the controversial evacuation of persons of Japanese ancestry from the West Coast. The next section outlines the Army's preparations for defense of the principal outlying possessions of Hawaii, Alaska, and the Canal Zone and includes an abbreviated account of the attack at Pearl Harbor and the Aleutian Islands Campaign, the only major ground operation to occur in the hemisphere during the war. The third section explains the creation of Army defenses to secure the Panama Canal in the south and the similar outpost line erected in the North Atlantic.

Governmental decisions to participate in the defense of Greenland and Iceland are explained as the culmination of American defensive measures by ground and air forces before the United States became a belligerent in World War II. Throughout, the authors stress the intimate relationship between American policy and the preservation of a North Atlantic lifeline to Great Britain, as well as the direct relationship between security of the Americas and the operations of the Royal Navy. Two other issues merit special mention. First is the tension between the natural desire to use a limited Army to defend the hemisphere in the opening days of the war and the need to use those same soldiers to train the large citizen army needed to pursue the war. Second is the evolution of the nation from a defensive to an offensive posture, specifically during the Coral Sea, Midway, and North Africa campaigns in view of the steadily declining threat of Axis attacks directed at the United States proper.

Key topics:
1. Transition from peacetime Army command and organization to wartime organization (Chs. II, III).
2. Problems in establishing unity of command among the services for hemisphere defense (Ch. IV).
3. The defensive problems facing the United States as a continental power isolated by two oceans, and the relationship between security of the homeland and offensive action in overseas theaters (Ch. III).
4. Defensive preparations in the outlying possessions of the United States during 1940 and 1941 (Chs. VII, IX, XII, XIII).
5. Acquisition, manning, and organization of new bases in the Atlantic and Caribbean in 1940-41 (Chs. XIV, XV).
6. Administrative processes leading to the administration's decision to evacuate resident enemy aliens and Americans of Japanese ancestry from the western states (Ch. V).
7. Fighting on the periphery of the American defensive perimeter in 1941-42, including Pearl Harbor, Alaska, and the Panama Canal Zone (Chs. VII, X, XI, XVI).
8. Establishment of wartime bases in Greenland and Iceland (Chs. XVII, XVIII, XIX, XX).