The U.S stock market has had a great run. Let me give you just a couple of numbers to ponder. On March 1, 2017, the S&P 500 index closed at an all-time high of 2,395.96. That represented a 254% gain from its close of 676.53 at the low point of the bear market on March 9, 2009. Alternatively, viewed from a more recent perspective, it also represented a 31% gain from its most recent low close of 1,829.08 back on February 11, 2016.Since March 1, the market has declined slightly, with the index closing at 2,328.95 on April 13, 2017. The U.S. market tailed off sharply into that close, apparently unnerved by a combination of the United States dropping the most powerful non-nuclear bomb ever used in combat on ISIS targets in Afghanistan as well as rapidly mounting tensions with North Korea. Given all of this, investors may rightly be concerned with finding a good balance between maintaining some exposure to the possibility for gains while at the same time ensuring at least a reasonable level of capital preservation. After all, if the markets experience a sudden, sharp selloff, paying attention to this now may help to allay investors’ fears while also preserving capital to take advantage of subsequent market recoveries.In this vein, take a look at the picture below. This picture covers the period from January 5, 2016 to the most recent market close. As can be seen, the red line represents the performance of the S&P 500 index, the blue line represents the performance of The ETF Monkey Vanguard Retirement Portfolio. This next picture features a tighter focus, namely YTD 2017.In both cases, the periods shaded in green represent market declines. In both cases, you will notice that this particular portfolio holds up very well during such periods.What is the structure and weighting of this portfolio? As it turns out, it is built from 7 ETFs. However, don't let this fool you. The portfolio is broadly diversified, including allocations to U.S. stocks and bonds, foreign stock, REITS and TIPS. Have a look.As can be surmised from the portfolio's name, it was built with the retirement investor in mind. However, the principles on which it is based could also prove advantageous to an investor who feels it might be appropriate to play a little defense right now.If you're interested in digging further into the details of the portfolio, feel free to take a look at the articles in which I detailed the setup of this portfolio, as well as The ETF Monkey 2016 Model Portfolio on which this retirement variant was based.I wish you all the best and, oh yeah, let's be careful out there.