Let's start with a blank Illustrator document. Select your Polygon Tool (nested under Rectangle Tool), and click once in the drawing area. A box will pop up and you can enter 3 for number of sides and anything you like for Radius. I used 0.6944 in.
Click OK and voilà! Your triangle. It will automatically be filled with whatever your foreground color is.
Select the triangle, and go to your Brush panel. Click on the little triangle in the upper right of the panel and select New Brush. From the resulting dialog box, select Pattern Brush and OK. In the next dialog box, give your brush a name. If your triangle had its point toward the sky, like mine did above, you'll want to check Flip Across. For the Fit, I used 'Stretch to fit.' This setting has the corners of each flag touching one another. If you select 'Add space to fit,' there will be a bit of space in between each flag. Choose whichever you like best! (P.S. it is fun to play around with the other settings and options here, so go ahead and give it a shot.) Click OK.
You will now see a representation of your new brush in the Brushes Panel and can start experimenting. Select your pen tool and draw a swooping curve. Make sure you have no fill, and then select the triangle brush from your palette.
Except, I think my triangles are a little too large. Double click on your brush in the Brushes palette, and reduce the size some. I went down to 70%. Click OK, and then Apply to Strokes.
Ta da! It's that easy to make a garland of flags. If you'd like to make them different colors, select your path, go to the Object menu at the top of the screen, then under Path, select Outline Stroke. You can then select each individual triangle with the Direct Selection tool and recolor it. I've done an ombré look below.
For another way to use your custom brush, select your Brush tool, rather than the Pen. Make sure your triangle brush is selected. If you drag a long curvy stroke out with the Brush tool, your flags will have more dimension and character to them. I picture them blowing in the wind.
Also reminds me of the scales on a dragon's back...
If you use the Brush tool and make short, choppy strokes, you will have a sort of confetti-like effect, drawing out one, beautifully imperfect triangle at a time.
This brush can lend itself to so many applications. What can you come up with? Please share!
P.S. For the garland image at the very top, I copied and pasted the flags into Photoshop as a Smart Object. I then added Drop Shadow, Bevel/Emboss, and a Texture Overlay.