The country spends among the most on defence in Southeast Asia and recent big purchases -- including submarines and tanks from China -- have drawn criticism in a state riddled with inequality and corruption.

Its generals have grabbed power a dozen times since 1932 with defence spending spiking in lockstep with each coup.

Last year the junta-picked National Legislative Assembly proposed a defence budget for 2019 of $7 billion, a $1 billion increase since the latest 2014 coup.

But Pheu Thai, linked to ex-premier Thaksin Shinawatra and the country's most popular political party, has vowed to end conscription and cut spending by 10 percent if it beats the odds and returns to power after the March 24 poll.

Junta chief Prayut Chan-O-Cha -- who hopes to become civilian premier under an army-aligned party after the elections -- on Monday justified the increase as necessary to upgrade obsolete gear.

"If we do not have it, our military quality cannot be compared with other countries," he told reporters.

Chan-O-Cha said the breakout of war was impossible to predict and in peacetime the army "fights against drugs and illegal entry" into the country.

Military conscription, he added, is "a duty of all Thai men".

The army runs an annual draft for 100,000 men each year -- all male citizens must participate at least once after they turn 21, but are exempted if they are students.

Thailand has not fought a full blown war in living memory, although border disputes have previously turned into bloody conflicts with Laos and more recently Cambodia.

The military has used its might more frequently inside the country, quelling street protests, staging coups or fighting insurgents in the Muslim-majority "Deep South", bordering Malaysia.

Army chief Apirat Kongsompong gave a coded slapdown to Pheu Thai's leadership over the proposal to cut military spending, insisting they should listen to a nationalist Cold War-era propaganda song, Nhak Paen Din.

Roughly translating as "Scum of the Earth", it was the anthem of paramilitary units who led a massacre of at least 41 students protesting the return of a military dictator in October 1976.

The #NhakPaenDin was top trending on Thai Twitter on Monday, a sign of the bitter memories it evokes, particularly among Thailand's pro-democracy movement.