How would you make the Armed Services more flexible in responding to present and future security challenges ?

Lt Col (Join to see), Sir, I think all the branches have already began to realign mission sets with smaller more agile deployable and home station missions while still keeping enough large mission capabilities to be able to still do large scale missions. The large scale may become an issue for long term supportability if the draw down in personnel continues.

Dear Lt. Col. Malone,
First, I believe we, Americans and warriors, need to stop giving a damn about what our 'critics' think of us. WE are the ones signing up and putting our lives on the line for our fellow Americans, our country, and our Allies.
Secondly, some things, like fighting Communism, should never change.
We should ALWAYS be committed to preserving freedom and liberty for ourselves and our children, against "ALL Enemies; foreign and domestic".
Third, before we can fight our enemies, we have to KNOW our enemies.
Aside from Communism, our enemies should be any who advocates or perpetrates oppression, violence, murder, and mayhem against society and nations.
Specifically, I am speaking about the Islamic State of Iraq and al-Sham, or 'ISIS' as it is simply known, but we should NOT relax our vigilance and look only to our soil and water for threats.
We are an AIR POWER, and for quite some portion of the last 20+ years, we have sought to reinforce our air superiority by reaching farther, to cyberspace, to protect our nation from security threats most Americans never stop to consider.
With the technology in computers that advanced with lightning-speed, we must seek to define and control it [the technology], which is how we'll negate security threats to the U.S. and her people; threats such as hackers or hostile agents who can utilize computers, computer programs, and their "information highway" to perpetrate attacks against us. To that end, the U.S. Air Force, and each military branch, should seek to identify threats that each of us can be harmed by without being aware of the harm.
Along with establishing more units to supervise cyberspace, we should encourage feedback from the experts... counter-hackers... to be able to formulate faster, more-effective security countermeasures. Why recruit a specialist if you're not going to take his/her advice?
Basically, I'm suggesting to create the 21st century equivalent of the 1950s Cold War survivalist:
people who anticipate major disruptions in or total collapse of the existing social order in
the future due to war, natural disaster, economic collapse, disease, divine retribution or other reasons, the latter being computer hackers who attack our nation's infrastructure.
"Know thy self, know thy enemy. A thousand battles, a thousand victories."Sun Tzu

I don't think closing so many bases overseas is the way to do it. All that does is extend the time it takes to respond to threats around the world. Smaller, more contingency focused first response units that are basically back filled by larger, sustainment focused units seems to make sense to me.