When I was an undergrad in the late 1970s it was taken for granted that appeasement was always a bad thing. I imagine that it is still the same today. I was surprised when I read British iconoclast diplomatic historian A.J.P. Taylor when he wrote that sometimes appeasement is a good thing in diplomacy. Then when I studied counter-insurgency theory it clicked. In counter-insurgency (COIN) warfare, as in dealing with mutinies, the idea is to defeat the enemy's military effort and then defuse the popular discontent that led to the rebellion in the first place. This was classically carried out by Britain in defeating the Arab Revolt in Palestine in 1938-39. Britain combined an alliance with Zionist forces, reinforcements from elsewhere in the Empire, and appeasement of Palestinian Arab grievances against Britain and the Zionists to end the rebellion. In the spring of 1939 Britain agreed to end land sales of Arab land to Zionists, limit Jewish immigration to 100,000 over five years and then limit it by giving the Arabs a veto over future Jewish immigration. This secured Britain relative quiet in Palestine during World War II--relative because this caused a revolt by Zionist extremists of the Lehi and Irgun. From this I took away that the key question as to whether or not appeasement would be successful was: Does the leader being appeased have finite limited goals that can be assuaged? Or does the leader have an appetite that grows with eating, as the French saying goes? Hitler is the classic example of the latter in the late 1930s and the British and French conservatives who attempted to appease him should have known this from his own book, Mein Kampf (MyStruggle) written while Hitler was in prison briefly for his 1923 attempted coup in Munich. Napoleon was another example of a leader who had expansive goals. Those who are good candidates for appeasement are nationalist leaders who have long-defined irredentist goals such as the Irish towards Northern Ireland or the Hungarians towards Transylvania.
So what about Putin? Are his territorial goals limited? Putin is no Communist, but he is also no democrat. He is a Russian nationalist whose long-term territorial goals seem to be the recovery of Russian territory under the late Russian Empire (Czarist, not Soviet) and lost since then. This means the Ukraine and Belarus, Kazakhstan and Central Asia, and the Baltic States, possibly even Finland. No naturally he has priorities. The core of the Russian Empire consisted of Russia, Belarus, and Ukraine--Great Russia, White Russia, and Little Russia. As Patrick Buchanan points out throughout the entire history of the American presidency from Washington until Clinton, Ukraine was part of the Russian Empire.
The next question to ask is: Can we reasonably reverse or resist the territorial ambitions of the leader at a reasonable cost? I never supported the inclusion of Ukraine or the Baltic States within NATO. I thought that they would be too difficult to defend because of their geographical location and that many established European countries ("Old Europe" in Rumsfeld's parlance) would resist going to war over Russian encroachment over its former territories. Because the Baltic States are now part of NATO they must be defended. When Britain and France went to war over Poland in September 1939 it was not so much to protect the independence of Poland as to defend the balance of power in Europe against Germany. This is why the West was willing to surrender control of Poland to Stalin in 1944-45. Stalin had the Red Army in Eastern Europe in 1944 and no one wanted to prolong World War II in order to defend Polish independence. The alternative was to make the Soviets pay for absorbing Eastern Europe into the Soviet Empire. This should be the approach towards Ukraine. Putin should understand that there will be a severe price to pay in terms of economic sanctions if he goes on to occupy eastern Ukraine.
We should then defend the Baltic States by deploying NATO troops in them and possibly even tactical nuclear weapons if the Balts are agreeable to this. If Putin goes ahead with his plans for Ukraine he may give NATO a real political purpose for the first time in over twenty years. We will also be back in a cold war, but one limited to Europe because of the limited exportability of Russian nationalism as an ideology.