If the Tagreans are the only civilization on their planet and they have never had contact with other planets then there is no explanation as to why they need such a heavily armed and aggressive military force.
However, many countries on Earth have had (some still have) strong military forces, to control their own people.
At the end when Carter is dialing the gate she doesn't press the symbol for the point of origin for Earth. This should have been the final symbol.
Colonel Ronson's rank insignia, the eagles on his shoulders, are upside down throughout the episode.
When the Stargate is being dialed at the end, you can hear the chevrons activating and the wormhole being opened. However, you can see clearly on screen that the Stargate is not opened.
While SG-1 is having supper, Chairman Ashwan comments on the food having been prepared "millions of light years" away. The Milky Way galaxy is 100,000 light years in diameter. The Prometheus didn't have inter-galactic capability at the time. The nearest large galaxy, Andromeda, is 2,000,000 light years away.