Politician, favored Restoration.

Killed in Bergen, Norway, in the attack on the Dutch East Indies fleet commanded by his cousin Edward, 1665. Apparently he was serving with his cousin with the idea of turning him to the Royalist and Catholic cause, and apparently he partially succeeded.

A rather macabe `experiment' was connected with his death and is described thus:

`John ... was a friend of the young and dissolute Lord Rochester [1] ... and of Henry Montagu, whose grandfather had died in 1644 a prisoner in the Tower in the hands of Parliament. The three joined the fleet together. Montagu was convinced that he would never return alive. John had the same presentiment, but was not so sure of its truth. Rochester was not troubled by the same anticipation, but had peculiar notions about the character of the soul which made him believe that communications from the dead could be arranged. Accordingly he and John entered into a formal agreement, ratified by religious ceremonies, that if either were killed he should appear and give the other a description of the future state, if there was one. The bond was put to the test on 2 August 1665, when John and Montagu were slain by the same cannon ball in an unsuccessful attack on some Dutch East Indiamen in the neutral port of Bergen. Needless to say, John failed to keep his post-mortem appointment and Rochester was bitterly disappointed.' (Wyndham).

Wyndham apparently is mistaken, it was Edward, not Henry, who was killed at Bergen. Lord Rochester became a poet of note, and left a letter describing the incident, as did others. Regarding the religious experiment, Rochester wrote: `Mr Montague would not enter into the bond.' Graham Green writes:

`There had been a rush to volunteer by the young aristocratic adventurers. Bergen meant for them... not death, wounds, or inglorious failure, but booty... Rochester was one of the poorest peers in the land and he courted one of the richest heiresses. Bergen might turn failure into success. ... with Rochester were Edward Montague and `another gentleman of quality'... Both had premonitions of death, Montague the more certain. ...

... Talbot had not properly completed his mission, the Governor was unready to betray the Dutch. ... Edward Montague was sent on shore with, it was rumoured, authority to offer the Governor the Garter... the Dutch prepared for resistance. ... their teams had been landed and placed ... in the forts. At dawn of 2 August Teddeman delayed no longer and opened fire. There was no room to manoeuvere... the yardarms stuck in the rocks; the wind... prevented the use of fire-ships... the forts opened fire upon the fleet. Teddeman did well in escaping without the loss of a ship. Four hundred men were lost... Among the dead were Montague and Wyndham.'(Greene).

Greene, who uses the e spelling, quotes a witness, a Dr. Burnet (who also uses the e spelling, as do Rochester's letters):

`Mr Montague, through he had such a strong presage in his mind of his approaching death, yet he generously stayed all the while in the place of greatest danger. The other gentleman signalized his courage in a most undaunted manner, till near the end of the action, when he fell on a sudden into such a trembling that he could scarce stand, and Mr Montague going to him to hold him up, as they were in each other's arms, a cannon ball killed him outright and carried away Mr Montague's belly, so that he died within an hour after.'(Burnet).

Greene notes: `It is the first event in Rochester's life of which we have an account in his own hand and the first which has left a mark on his poetry [1] ...'.

Sources:
[DNB]
A Family History, Wyndham.
Lord Rochester's Monkey.

http://www.montaguemillennium.com/familyresearch/h_1665_edward.htm