I understand the sentiment but whatever reaction Obama had it all came too late after our adversaries created reality on the ground and it'd be next to impossible to dislodge them.
Take the case of South China Sea islands now occupied by China. If the moment they started that he moved battlegroups to the area and put a diplomatic pressure, the China might have backed down. Now they practically own the island. Now you'd have to start a war to dislodge them.
That's why is a naive pussy. He could have taken similar actions in Crimea but he didn't we were much more stronger then than now.
Generally it's foolish to start confrontations in this realities now. So the best option is to make peace where possible and work to not allow them further ground.