Undercover Ops is a side-scrolling stealth game from Arcade Bomb. You are behind enemy lines with a knife, and your task is to deal with the soldiers quietly, one at a time.
Controls. Left and right arrow keys move you, the up arrow climbs or grips surfaces as you advance, the down arrow takes you back down, and the space bar attacks. The main menu has an Instructions page if you need a reminder.
Approach. Timing is everything. Wait until a guard looks away, come up behind him and strike. Taking enemies on one at a time is safer than rushing a group.
It is a short, tense game, and a good change of pace from puzzles. Arkanoid Flash is another Flash-era classic on this site.