Ok - did some lightining research.
s.3 SAF Act:
3. The following persons shall be persons subject to military law
and this Act shall, subject to such exclusions or variations in
relation to non-uniformed servicemen as may be prescribed, apply to
them:
(a) regular servicemen and full-time national servicemen in the
Singapore Armed Forces from the time their liability to report for
enlistment or service arises until such time as they are lawfully
discharged or released;
(b) operationally ready national servicemen from the time that they
are ordered to report for enlistment or service, whether they have
complied with such order or not, and while in uniform or performing
service in the Singapore Armed Forces or the Peoples Defence Force
pursuant to section 14 of the Enlistment Act (Cap. 93), and while
called out for mobilised service pursuant to sections 16 and 17 of
that Act or when called out to duty in aid of the civil
power;
...
So NSmen are only liable under the act when they have been
recalled or mobilised. AFAIK, There is no direct prohibition on the
joining of a foreign army in the act, unless that army happens to
be the enemy. (Look under mutiny, AWOL, and
Desertion.)
-- Treason --
SINGAPORE (Penal Code, Chapter VI)
CHAPTER VI
OFFENCES AGAINST THE STATE
Waging or attempting to wage war or abetting the waging of war
against the Government. 121. Whoever wages war against the
Government, or attempts to wage such war, or abets the waging of
such war, shall be punished with death, or with imprisonment for
life, and shall also be liable to fine.
Illustration
A joins an insurrection against the Government. A has committed the
offence defined in this section.
....
Offences against authority. 121B. Whoever compasses, imagines,
invests, devises or intends the deprivation or deposition of the
President from the sovereignty of Singapore, or the overawing by
criminal force of the Government, shall be punished with
imprisonment for life, and shall also be liable to a fine.
Abetting offences under section 121A or 121B. 121C. Whoever abets
the commission of any of the offences punishable by section 121A or
121B shall be punished with the punishment provided for those
offences.
Intentional omission to give information of offences against
section 121, 121A, 121B or 121C by a person bound to inform. 121D.
Whoever knowing or having reason to believe that any offence
punishable under section 121, 121A, 121B or 121C has been committed
intentionally omits to give any information respecting that offence
which he is legally bound to give, shall be punished with
imprisonment for a term which may extend to 7 years, or with fine,
or with both.
Collecting arms, etc., with the intention of waging war against the
Government. 122. Whoever collects men, arms or ammunition or
otherwise prepares to wage war, with the intention of either waging
or being prepared to wage war against the Government, shall be
punished with imprisonment for life or imprisonment for a term not
exceeding 10 years, and shall also be liable to fine.
Concealing with intent to facilitate a design to wage war. 123.
Whoever by any act, or by any illegal omission, conceals the
existence of a design to wage war against the Government, intending
by such concealment to facilitate, or knowing it to be likely that
such concealment will facilitate, the waging of such war, shall be
punished with imprisonment for a term which may extend to 10 years,
and shall also be liable to fine.
....
Waging war against any power in alliance or at peace with
Singapore. 125. Whoever wages war against the government of any
power in alliance or at peace with the Government, or attempts to
wage such war, or abets the waging of such war, shall be punished
with imprisonment for life, to which fine may be added; or with
imprisonment for a term which may extend to 7 years, to which fine
may be added, or with fine.
Committing depredation on the territories of any power in alliance
or at peace with Singapore.
126. Whoever commits depredation, or makes preparations to commit
depredation, on the territories of any power in alliance or at
peace with the Government, shall be punished with imprisonment for
a term which may extend to 7 years, and shall also be liable to
fine and to forfeiture of any property used, or intended to be
used, in committing such depredation, or acquired by such
depredation.
Receiving property taken by war or depredation mentioned in
sections 125 and 126. 127. Whoever receives any property knowing
the same to have been taken in the commission of any of the
offences mentioned in sections 125 and 126, shall be punished with
imprisonment for a term which may extend to 7 years, and shall also
be liable to fine and to forfeiture of the property so
received.
....
Again, deosn;t seem to be a prohibition on the joiining of
a foreign army. There are, however, some sections that may be of
concern. Eg. When the army you join goes to war against Singapore,
or when that army attacks a nation at peace with
singapore.
--------
Need more reserach.