Evaluate the sum $1 + 2 - 3 + 4 + 5 - 6 + 7 + 8 - 9 \cdots + 208 + 209 - 210.$
1. **Group the terms in sets of three:**
   We observe that the terms can be grouped as follows:
   \[
   (1 + 2 - 3), (4 + 5 - 6), (7 + 8 - 9), \ldots, (208 + 209 - 210)
   \]
   Each group contains three terms.

2. **Evaluate the sum of each group:**
   Let's calculate the sum of the first few groups to identify a pattern:
   \[
   1 + 2 - 3 = 0
   \]
   \[
   4 + 5 - 6 = 3
   \]
   \[
   7 + 8 - 9 = 6
   \]
   \[
   10 + 11 - 12 = 9
   \]
   We notice that the sum of each group forms an arithmetic sequence with the first term \(0\) and a common difference of \(3\).

3. **Determine the number of groups:**
   Since the sequence ends at \(210\) and each group contains \(3\) terms, the total number of groups is:
   \[
   \frac{210}{3} = 70
   \]

4. **Identify the last term of the arithmetic sequence:**
   The last group is:
   \[
   208 + 209 - 210 = 207
   \]
   Therefore, the last term of the arithmetic sequence is \(207\).

5. **Sum the arithmetic sequence:**
   The sum of an arithmetic sequence can be calculated using the formula:
   \[
   S_n = \frac{n}{2} \cdot (a + l)
   \]
   where \(n\) is the number of terms, \(a\) is the first term, and \(l\) is the last term. Here, \(n = 70\), \(a = 0\), and \(l = 207\):
   \[
   S_{70} = \frac{70}{2} \cdot (0 + 207) = 35 \cdot 207
   \]

6. **Calculate the final sum:**
   \[
   35 \cdot 207 = 7245
   \]

The final answer is \(\boxed{7245}\).