Brown University estimates the US Wars on Terror will be $5.9 trillion through 2019. This includes direct war and war-related spending and obligations for future spending on post 9/11
US war spending will continue to grow. The Pentagon currently projects $80 billion in Overseas Contingency Operations (OCO) spending through FY2023. Even if the wars are ended by 2023, the US would still be on
track to spend an additional $808 billion to total at least $6.7 trillion, not including future interest costs. The costs of war will likely be greater than this because the number of veterans will grow until all wars are ended. Veterans benefits and disability spending and the cost of interest on borrowing to pay for the wars, will make up more and more of the costs.
Even if all operations are terminated in Pakistan, Afghanistan, Iraq and Syria, it seems the interest payments and Homeland Security will continue. The threat of middle east terrorism to the USA seems to be a permanent condition.
Permanent Extra Costs for the USA
Could Homeland Security be provided at lower cost? If costs are reduced then every four years the spending could be $600 billion instead of $800 billion. This is $1.5 trillion every decade versus $2 trillion every decade.
Until the US pays down all of its current debt then $1.5 trillion per decade in extra interest and Homeland Security is a permanent extra budget cost.