The US being able to declare war during the Collect Income Phase is an EXCEPTION to the general rule for everyone. It does not override the Declaration of War rule.
Pacific 1940 2nd Edition Rules
PAGE 11 (Sidebar) (Bold added by me.)
Declaring War
If there are no restrictions currently keeping you from being at war with a power on the other side (see “The Political Situation,” page 8), you may declare war on that power. War must be declared on your turn at the beginning of the Combat Move phase, before any combat movements are made, unless otherwise specified in the political rules. An actual attack is not required. Once a state of war is entered into, all territories and sea zones controlled by or containing units belonging to the power or powers on which you declared war instantly become hostile to your units, and the normal restrictions of moving into or through hostile
spaces apply, with one exception. During your Combat Move phase in which you entered into a
state of war, your transports that are already in sea zones that have just become hostile may be loaded in those sea zones (but not in other hostile sea zones). In effect, transports may be loaded in their initial sea zones for amphibious assaults before war is declared, while the sea zone is still friendly.
Declarations of war are either “provoked” or “unprovoked.” A declaration of war is considered to be provoked either when the declaration is made in reaction to the direct aggression of another power or when a political rule allows the power to declare war due to a specific action being taken by another power. For example, if the United Kingdom declares war on Japan, a subsequent declaration of war by Japan on the United Kingdom (which is technically unnecessary, as the two powers are already at war) would be considered provoked.
PAGE 9
UNITED STATES
The United States begins the game neutral. It may not declare war on Japan unless Japan first declares war on it or makes an unprovoked declaration of war against the United Kingdom or ANZAC. However, if the United States is still not at war with Japan by the Collect Income phase of its third turn, it may declare war on Japan at the beginning of that phase. This is an exception to the rules for declaring war (see “Declaring War,” page 11), which may normally be done only at the beginning of the Combat Move phase.
PAGE 37
Political Situation: The United States begins the game at war with no one. In addition to the normal restrictions (see “Powers Not at War with One Another,” page 15), while it’s not at war with Japan, the United States may not move any units into or through China or end the movement of its sea units in sea zones that are adjacent to Japan-controlled territories. While
not at war with Germany or Italy, the United States may end the movement of its sea units on the Europe map only in sea zones that are adjacent to U.S. territories, with one exception: U.S. warships (not transports) may also conduct long-range patrols into sea zone 102. The United States may not declare war on any Axis power unless an Axis power either declares war on it first or captures London or any territory in North America, or Japan makes an unprovoked declaration of war against the UK or ANZAC, after which it may declare war on any or all Axis powers on its following turn. However, if it’s not yet at war by the Collect Income phase of its third turn, the United States may declare war on any or all Axis powers at the beginning of that phase. This is an exception to the rules for declaring war (see “Declaring War,” page 12), which may normally be done only at the beginning of the Combat Move phase.
Normally a Power is not allowed to declare war until the Combat Move Phase. The US has a specific exemption from this rule that it can begin to declare war on the Collect Income phase of the 3rd Turn instead of having to wait until the Combat Move Phase of the 4th Turn. Just because the US does not use this exemption does not remove its ability to now declare war on any future turns beyond Turn 3 during the Combat Move Phase. It has met and completed its restrictions by waiting until at least the Collect Phase of the 3rd Turn or responded to an unprovoked attack before declaring war. Now that those restrictions are removed they may declare war.
