When asked by Senator McCain if he supported a permanent military presence in the Baltics, General Mattis looked down briefly and then reluctantly responded, "I do sir". Gen. Mattis also agreed with Sen. McCain that Russia and China currently posed the greatest threat to the "World Order" set up after WW2.
If Russia had any hopes of working with a Trump-controlled US, General Mattis testimony during his senate conformation hearings may have just sunk them. During the presidential campaign, President-elect Trump questioned the extent of US and NATO's involvement overseas. But with General Mattis appointed as Secretary of Defense, it is looking like US foreign policy may continue on a similar course.
The Mattis testimony is very immediately dangerous for the US. If Russia and China were holding out to see if they could work with Trump, if it looks like Trump is going to support continued NATO "deterrence" in Eastern Europe, then Russia and China may decide to "act" now during the confusion of a US presidential administration transition.