Yes, NATO countries do not spend equally on defense. NATO countries varied quite significantly in what they contributed to the efforts of the past--Bosnia, Kosovo, Afghanistan and Libya. But there are a few basic ideas that need to be addressed.
First, the Canadians and Europeans do bring something to the table--it is not just a US effort with a NATO flag. None of the members of NATO besides the U.S. really had much at stake in Afghanistan. They all showed up, and damn near all of them paid a significant price in blood and treasure. Despite the rhetoric, the various provincial action plans (the Helmand Plan and so on), and all the rest, these countries were there because they were fulfilling what they saw as an Article V commitment--that an attack upon one happened, and that they were obligated by norms, by affinity and by self interest to support that country. They bought that Afghanistan was a response to 9/11. They did not buy Iraq in the same way, although some showed up anyway.
For much of the war, Americans were outnumbered on the Afghanistan battlefields by Canadians and Europeans. Only as Obama surged did the balance of effort tilt. While the U.S. was off starting unnecessary wars, its allies were holding the line more or less.
So, when people ask: will the NATO members show up when needed, the answer is yes. Indeed, whenever the alliance is threatened, the members come together because they are more about the alliance than pretty much anything else--certainly more than Bosnia or Kosovo.
Second, the US spends some of its defense dollars on European security (much of the US defense budget is aimed elsewhere) and provides far more to the alliance than many of the members but IT IS NOT OUT OF ALTRUISM. The US has much at stake in European stability. A few World Wars and a Cold War proved that. So, what the US spends in Europe for European security is due to American interests and not just being a good pal, a sucker to be exploited by the Euros. Much of our prosperity since WWII has been built on North American partnership with Europe. The pivot to Asia was built on the premise that Europe needed less (but still some) American effort. With Russia's aggression, the pivot will not be quite as significant.
Anyhow, this post is a reaction to the ideas that the allies are completely flaky and that the US is engaged in Europe due to its charitable nature. Neither of these ideas have much of a basis in reality. Yes, I co-wrote a book on how countries varied in their efforts in Afghanistan, but we cannot ignore that they all showed up and the ones that we tend to criticize--the Germans, the French, and the Italians--were among the biggest force providers, bled quite a bit, and got damn near no credit for it.