except as provided in subparagraphs (B) and (C), pursuant to regulations prescribed by the Secretary of Defense;

for persons performing service to the Federal Emergency Management Agency under section 327 of the Robert T. Stafford Disaster Relief and Emergency Assistance Act (42 U.S.C. 5165f) and as intermittent personnel under section 306(b)(1) of such Act (42 U.S.C. 5149(b)(1)), by the Administrator of the Federal Emergency Management Agency as described in sections 327(j)(2) and 306(d)(2) of such Act (42 U.S.C. 5165f(j)(2) and 5149(d)(2)),1 respectively; or

for intermittent disaster-response appointees of the National Disaster Medical System, by the Secretary of Health and Human Services as described in section 2812(d)(3)(B) of the Public Health Service Act (42 U.S.C. 300hh–11(d)(3)(B)).

ordered to or retained on active duty under section 688, 12301(a), 12301(g), 12302, 12304, 12304a, 12304b, or 12305 of title 10 or under section 331, 332, 359, 360, 367, or 712 2 of title 14;

ordered to or retained on active duty (other than for training) under any provision of law because of a war or national emergency declared by the President or the Congress, as determined by the Secretary concerned;

ordered to active duty (other than for training) in support, as determined by the Secretary concerned, of an operational mission for which personnel have been ordered to active duty under section 12304 of title 10;

ordered to active duty in support, as determined by the Secretary concerned, of a critical mission or requirement of the uniformed services;

called into Federal service as a member of the National Guard under chapter 15 ² of title 10 or under section 12406 of title 10; or

ordered to full-time National Guard duty (other than for training) under section 502(f)(2)(A) of title 32 when authorized by the President or the Secretary of Defense for the purpose of responding to a national emergency declared by the President and supported by Federal funds, as determined by the Secretary concerned.

the employer’s circumstances have so changed as to make such reemployment impossible or unreasonable;

in the case of a person entitled to reemployment under subsection (a)(3), (a)(4), or (b)(2)(B) of section 4313, such employment would impose an undue hardship on the employer; or

the employment from which the person leaves to serve in the uniformed services is for a brief, nonrecurrent period and there is no reasonable expectation that such employment will continue indefinitely or for a significant period.

any reemployment referred to in paragraph (1) is impossible or unreasonable because of a change in an employer’s circumstances,

any accommodation, training, or effort referred to in subsection (a)(3), (a)(4), or (b)(2)(B) of section 4313 would impose an undue hardship on the employer, or

the employment referred to in paragraph (1)(C) is for a brief, nonrecurrent period and there is no reasonable expectation that such employment will continue indefinitely or for a significant period,

In the case of a person who is absent from a position of employment for a period of any length for the purposes of an examination to determine the person’s fitness to perform service in the uniformed services, by reporting in the manner and time referred to in subparagraph (A).

In the case of a person whose period of service in the uniformed services was for more than 30 days but less than 181 days, by submitting an application for reemployment with the employer not later than 14 days after the completion of the period of service or if submitting such application within such period is impossible or unreasonable through no fault of the person, the next first full calendar day when submission of such application becomes possible.

In the case of a person whose period of service in the uniformed services was for more than 180 days, by submitting an application for reemployment with the employer not later than 90 days after the completion of the period of service.

A person who is hospitalized for, or convalescing from, an illness or injury incurred in, or aggravated during, the performance of service in the uniformed services shall, at the end of the period that is necessary for the person to recover from such illness or injury, report to the person’s employer (in the case of a person described in subparagraph (A) or (B) of paragraph (1)) or submit an application for reemployment with such employer (in the case of a person described in subparagraph (C) or (D) of such paragraph). Except as provided in subparagraph (B), such period of recovery may not exceed two years.

Such two-year period shall be extended by the minimum time required to accommodate the circumstances beyond such person’s control which make reporting within the period specified in subparagraph (A) impossible or unreasonable.

the person’s application is timely;

the person has not exceeded the service limitations set forth in subsection (a)(2) (except as permitted under subsection (c)); and

the person’s entitlement to the benefits under this chapter has not been terminated pursuant to section 4304.

Except as provided in subparagraph (B), the failure of a person to provide documentation that satisfies regulations prescribed pursuant to paragraph (2) shall not be a basis for denying reemployment in accordance with the provisions of this chapter if the failure occurs because such documentation does not exist or is not readily available at the time of the request of the employer. If, after such reemployment, documentation becomes available that establishes that such person does not meet one or more of the requirements referred to in subparagraphs (A), (B), and (C) of paragraph (1), the employer of such person may terminate the employment of the person and the provision of any rights or benefits afforded the person under this chapter.

An employer who reemploys a person absent from a position of employment for more than 90 days may require that the person provide the employer with the documentation referred to in subparagraph (A) before beginning to treat the person as not having incurred a break in service for pension purposes under section 4318(a)(2)(A).