Those coming against Christ were a mixed group of Gentiles and Jews (Psalms 2:1-2), who hated each other but were united against the One Who came to reconcile both to each other by the sacrifice of His body (Eph 2:11-16).

Psalms 2:1-2  Why do the nations conspire and the peoples plot in vain? The kings of the earth take their stand and the rulers gather together against the LORD and against his Anointed One.

Ephesians 2:11-16  Therefore, remember that formerly you who are Gentiles by birth and called “uncircumcised” by those who call themselves “the circumcision” (that done in the body by the hands of men)—remember that at that time you were separate from Christ, excluded from citizenship in Israel and foreigners to the covenants of the promise, without hope and without God in the world.  But now in Christ Jesus you who once were far away have been brought near through the blood of Christ.  For he himself is our peace, who has made the two one and has destroyed the barrier, the dividing wall of hostility,  by abolishing in his flesh the law with its commandments and regulations. His purpose was to create in himself one new man out of the two, thus making peace,  and in this one body to reconcile both of them to God through the cross, by which he put to death their hostility.

During the Feast days, the Romans would house a cohort of soldiers (a tenth of a legion, or about 600 men) in the fortress of Antonia which adjoined the Temple, to prepare for a possible uprising among the numerous pilgrims. Judas did not receive the whole cohort, but probably a detachment of it. However, the Romans could sometimes use surprisingly large numbers of soldiers to accomplish their tasks, as they did to transport Paul to Caesarea in (Acts 23:23) and here it was night and they were going to arrest the leader of a potentially large band of followers who could cause a riot, so the number of soldiers was probably quite large.

Acts 23:23  Then he called two of his centurions and ordered them, “Get ready a detachment of two hundred soldiers, seventy horsemen and two hundred spearmen to go to Caesarea at nine tonight.

The Jewish authorities had probably brought in the Romans as soon as possible after deciding on their course of action. Only the Romans could carry out a death sentence and that is exactly what the Jews wanted for Jesus. Also, the temple guards had failed at their last attempt to arrest Jesus (7:44-46) and the Pharisees and Sadduccees would undoubtedly want to make sure that a failure did not occur this time. The Romans would have been unlikely to refuse such a request from the high priest with passions running so high at the Passover Feast and the looming possibility of Jesus resisting arrest and gathering an army of excited Galileans around Him.

John 7:44-46  Some wanted to seize him, but no-one laid a hand on him. Finally the temple guards went back to the chief priests and Pharisees, who asked them, “Why didn’t you bring him in?”  “No-one ever spoke the way this man does,” the guards declared.

The rather large group of Roman soldiers, temple guards, and Judas in the lead probably stopped first at the house (tradition holds that it was the house of Mark’s parents) where Jesus had celebrated the Passover meal. Learning that He had left with His disciples 2 or 3 hours earlier, Judas next led the small army to the place he knew so well: Gethsemane.

They were armed with lanterns, torches, and weapons. The first two were because they expected Jesus to try to escape and even though there was a full moon, He could easily slip away in the gloom of night unless they brought sufficient lighting. The weapons were in case of resistance on the part of His followers. Imagine the sight from the Garden as the very large detachment of soldiers and Jewish officials with their lanterns and torches wound their way up the path in the darkness of the night.