Base mission is similar to something like the Battlefield games. Starting at one point, your team must move forward to gain control of a certain number of other points (either 3 or 5) on the map. Your goal is to either take control of all the bases or have more bases captured when time runs out.

You get points (both regular and Drebin points) for capturing bases. For regular points, only the capture itself seems to count; for Drebin points, capturing the base gets you a big boost, but you also get 250 DP every few seconds that you're standing on a base (either to capture it or defend it). For this reason, this game mode is one of the most free with Drebin points, so expect a lot of high-level weapons like RPGs, XM8s, and grenade launchers.

Depending on the number of bases and the map, Base Mission can vary wildly in strategy. For example, in Urban Ultimatum, the starting bases are on top of the map's two tall buildings. When there are three bases, all the bases within view of each of the two towers and people taking them are very vulnerable to sniper fire. With five bases, the two other bases are in covered areas (in an alley and in a grassy corner, respectively) and are out of range of most snipers.

Because of the ratio of players to control points (max of 8 per team compared to 3/5 control points) most of the battles you fight will be usually between groups of 1-2 people. Therefore, individual combat plays an important role. If you're attacking a point, drop some grenades on it before you move forward, because the small area that needs to be defended is also roughly the size of a grenade blast. If you're defending, try to take out enemies from far away as soon as you see them and if you hear the telltale clink of a grenade landing, duck for cover. You don't need to stay on the point; you're trying to fill up a bar, essentially, and the bar will stay at the same point even if you get off the point. Get back on immediately when it's safe, though.

The other unique thing about Base Mode is SOP Destabilizers (SOP DESTAB). These items cost 5000 Drebin points and serve as a primary weapon (leaving you with only handguns and grenades to defend yourself). Their purpose is that, if the item is brought to the enemy's main base (the one you can't capture) and planted by pressing R1 and L1 together, the entire enemy team will enter a zombie-like state, convulsing, shaking, and unable to move. This lasts for 10-20 seconds, and is a good time for your team to go after bases. The effect is dispelled upon respawn, so killing a zombie-like enemy will return them to normal when they respawn. Both teams receive a notice when someone buys an SOP DESTAB, so if you hear that the enemy has one, have at least one person fall back and defend the main base (invest in claymores or other mines). If you have one, you'll have to make your way to the enemy base as stealthily as possible, because once they realize that you have it (people who have their sidearms out are usually identifiable as SOP DESTAB carriers) they'll go after you hard. This is a true test of stealth, but you can't take too long either because one person being invested in getting the SOP DESTAB to the enemy base is one person who isn't fighting for the other bases. Thus, move cautiously but quickly. Coordinate with your teammates to pressure an area on the side of the map you aren't going through.

Metal Gear Online Walkthrough
Metal Gear Solid 4 Walkthrough