OK, I'll give you some straight answers as to why that's not going to happen, even if you were president tomorrow:
1- End the Wars. Actually, the wars are ending. But let's say tomorrow is your first day in office. Your order is "Redeploy all the units". The CJCS says "Yes, sir". First they need some time to come up with a plan on how to do what you want. So MINIMUM 60 days. Ever tried to get a family of five in the car for a 5-day road trip?? How many hours did that take? OK, now multiply that by 50,000. Moving a force the size of what we have is not a small feat when it's in a land-locked country halfway around the world and we can't just drive down to the coast and hop on a boat. So to make sure it's done right and we don't give $20B worth of stuff to the Taliban when we leave, a plan is a good thing.
So then they come back with the plan, and say it will be 18 months. You lose your mind and say you want it done NOW (you are the President after all!). The CJCS brings in his Intel guys, who give you an hour long brief on the complexities and fragilities of the Afghan society, and how just leaving out of the blue will destroy all the progress made thus far, result in thousands of Afghan deaths due to the resulting civil war, create a resurgence of the Taliban, etc.. Most presidents at this point realize that these are ACTUAL lives that hang on their personal decision (think the picture of Kennedy in the Oval Office during the Cuban Missile Crisis). No longer an armchair exercise, they realize that there has to be a logical framework for the withdrawal. But like Iraq, it happens, because you are the boss. Just on a timeline tempered with reality and experience. Common Sense Ending...
But for arguments sake, let's say you are fanatical about this (you are the President after all!). You give direct that every available mode of logistics will be used immediately to remove US troops from Afghanistan. OK fine they say, and leave. The CJCS hands in his resignation, as his advice is no longer useful to you. Political mayhem ensues, stuff gets leaked to Congress/the Press, and you spend so much of your time dealing with that you can't keep track of the withdrawal.
You threaten to fire all not obeying your orders, those below you come with briefings showing how they are making progress as best they can, you don't have a clue how logistics works, so you don't know if they are lying or not. So you fire a couple just for good measure... briefings get more and more 'controlled'. Troops end up taking about 24 months to withdraw because of all the mess you made.
So let's say you veto the spending bill. Great idea! Resources are what drives DC. So now there is no funding for the war effort. Pentagon comes to a grinding halt. Problem is that there are still troops in the field (remember land-locked Afghanistan?), who are now dying because of lack of ammunition that you refused to buy them. Pictures of dead GI's come back home. Oh wait, now suddenly your veto gets overridden by Congress.
But you aren't done yet, you use yet more executive power to stop spending any DoD funds. More GI's die. Congress has now had enough, so has the American people, and you are the first to be Impeached/Convicted. And the Brits aren't fond of you either (remember it's a Coalition over there)
(The next 2 are easier)
2- End of Department "X". Which one? Defense? Education? State? Health and Human Services? Yep, you can slay an entire department as President. Problem is that in most areas of government, there is SOME good being done. So it's pretty unlikely you can just kill the whole thing without crippling a vital service people need. OK, no problem... we'll just carve out the fat, right? Trouble is that it's very hard to estimate how many people any department really needs if you aren't in that department (just how many people does it take to keep track of Social Security Numbers, I don't have a clue) And almost no one is coming to come brief you that they need fewer people (and they would benefit how??). So in frustration, you decide to issue a 10% funding cut across the board. So those few naive departments (headed by people like you, no offense) that decided to right-size their manning to exactly what they needed and are proud of how lean they are get swacked another 10% and now are all working 12 hour days with no extra pay. Naive people learn the bureacratic way. (See, they didn't start off that way, they are often made they why by their leadership)
The biggest 'slash' I've seen recently is DefSec Gates recently killed "JFCOM" with is a whole command in the military. Couple thousand people I think? Honestly, that was impressive.
3- Eliminate the Deficit: OK, first off, there is no such thing as a line-item veto. Why? Because Congress won't pass it. So your only tool as President is to veto the entire budget. That's it. Let's say that's exactly what you do. So now the government continues to run on a "Continuing Resolution Authority" (which basically says spend like it was last year's budget) until one gets passed. Congress goes back, and argues for a while, but because everyone has their pet projects and need to get re-elected, they make some cuts, but in your opinion, not enough and not in the right places. Again, veto is your only option. And each veto will cost 30-60 days until you see another budget come up. And in the end, whose fault will it be that the budget isn't signed in the public's eye?? Yours. So you'll lose, because Congress won't be motivated to do what you want. They'll just pass budgets until your approval ratings get so low they can impeach you.
So those are real answers why it's not that simple. The President is the most powerful person in the world, but steering the Titanic takes cunning/skill, not just brute force.
