I'm assuming you mean an old time army, not a modern one.

The Himalayas stretch east-west across the northern edge of the Indian subcontinent, and are basically uncrossable north-to-south. On the east end, you pretty much have to go around them through Indochina (Laos, Thailand, Burma) to get into the eastern part of India (Arunachal Pradesh).

On the west, you would have to go similarly far to the south to cross the mountains into West Pakistan, except that there is a pass through the mountains which makes it much easier. The Khyber Pass connects Afghanistan to northern Pakistan, and is the route through which people have entered and exited the Indian subcontinent since antiquity.

People have known about the Khyber Pass for a very long time. This is how neolithic farmers entered the subcontinent 8,000 years ago. This is how Indo-European language speaking people entered 4,000 years ago. It was part of the ancient silk road carrying trade and people. Alexander crossed into India through the Khyber Pass. Indian trade with Mesopotamia and Rome went through it. It's how the Islamic invasions of India from central Asia and Iran happened.

So the Himalayas are not an absolute barrier, but historically they have prevented casual contact between people on opposite sides. They were not a barrier to well-organized trade caravans or to a determined army, but they did require that the crossing be made in a specific place, i.e., the Khyber Pass.

It's possible to get into the Indian subcontinent in other ways, for example, through Burma in the east, or through southern Iran - follow the coast to Karachi - these were not favored routes for armies. You would have to cross hundreds of miles of very sparsely populated and almost barren lands on the west, or pass through some dense jungle on the east. This was hard to do for armies in the past because of logistics. By the time you got to any large populated centers in India worth looting, you would have a supply chain thousands of miles long through inhospitable terrain.