All 4 Uses

phase
in
Flags of Our Fathers
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At the conclusion of the second landing, the troops immediately reboarded the transports and headed for their final exercise of the Pendleton phase: an assault on what their officers now called "Pendleton Island," and described to them as "a strongly held advance air base of the Japanese in the Western Pacific—a base that menaced U.S. forces."†
Chpt 5.
It was during these final phases of training, in July of 1944, that a distinguished visitor began to make himself visible at Pendleton.†
Chpt 5. *
And so Blankenburger was surprised when, near dawn, he heard Mike say quietly: "I'm not coming back from this one, Ed." Over 70,000 Marines—the 3rd, 4th, and 5th Divisions—massed in the ships that had finally arrived at Island X, ready to hit the narrow two-mile beach in successive phases.†
Chpt 7.
As the tour neared the final phase of its eight-week loop around America, John and Rene could not have fully appreciated the scale of what they had accomplished.†
Chpt 16.

Definitions:

(1) (phase as in: a critical phase)
a period of time or a stage during the development of something
(2) (phase as in: she's going through a phase)
a period of time when someone exhibits unusual behavior
(3) (phase as in: phased out over three years)
make a change gradually (phase in = gradually add; phase out = gradually removed)
(4) (meaning too rare to warrant focus)
More specialized and less common senses of the word are found in astronomy, physics, chemistry, and zoology. Consult a comprehensive dictionary if you wish to see those. All senses have to do with stages or periods of time.